The smallest full-featured build I managed to get was my Lego PC ( Lego PC - Traditional Damascus Theme ).
It is 10"x10"x6" and has 5.25" and 3.5" and a DVD-RW drive and a sound card.

The key is to find the tinyest 386 motherboard first. Ideally, the more it integrates stuff the better (floppy controller, IDE controller, maybe even VGA).. but for 386 that would be very rare on generic boards. Only OEM boards tended to integrate such features, and they're usually quite big..

I still think it's achievable, but I say the mobo is the starting point. A combo floppy drive helps keep things tidy. Use a Disk-On-Module or CF card for HDD.

For housing it, this will be totally up to your skills and materials. I chose Lego because it was both interesting and very flexible. But it could have been wood, metal sheets, acrylic, 3D-print, ...etc.

I'm not much into the super tiny weecee stuff (even though it's cool) because they don't have floppy drives nor expansion capabilities (ISA/PCI slots), and I like to have that in my machines.

PC Partner XL is a good option, but difficult to find. I have seen similar generic cases as well out in the wild.

I posted about the XL system here.

The high-integration slimline desktop form factor was pretty popular back in the day... An example is this CompuAdd unit -

wbahnassi wrote on Yesterday, 10:46:

The smallest full-featured build I managed to get was my Lego PC ( Lego PC - Traditional Damascus Theme ).
It is 10"x10"x6" and has 5.25" and 3.5" and a DVD-RW drive and a sound card.

Wow. That is seriously cool. For several reasons. The fact it's a piece of art. The slimline optical drive. The combined format floppy drive. The Pico PSU. I am very impressed with that build. Well done.

wbahnassi wrote on Yesterday, 10:46:

The key is to find the tinyest 386 motherboard first. Ideally, the more it integrates stuff the better (floppy controller, IDE controller, maybe even VGA).. but for 386 that would be very rare on generic boards. Only OEM boards tended to integrate such features, and they're usually quite big..

Yeah, that makes sense. I will see what I can find.

wbahnassi wrote on Yesterday, 10:46:

I still think it's achievable, but I say the mobo is the starting point. A combo floppy drive helps keep things tidy. Use a Disk-On-Module or CF card for HDD.

Yeah, I was thinking of using one or two 512Mb CF cards. Those combo floppy drives seem more expensive than I can justify. £200 is the cheapest I can see right now, and its sold as untested (looks in a terrible state - like it was stored outside and rusted).

wbahnassi wrote on Yesterday, 10:46:

For housing it, this will be totally up to your skills and materials. I chose Lego because it was both interesting and very flexible. But it could have been wood, metal sheets, acrylic, 3D-print, ...etc.

I have a small workshop in the garage where I can cut wood, or craft things out of aluminium. A 3D printer is on my shopping list. I am waiting until Black Friday. Hoping for a deal one.

wbahnassi wrote on Yesterday, 10:46:

I'm not much into the super tiny weecee stuff (even though it's cool) because they don't have floppy drives nor expansion capabilities (ISA/PCI slots), and I like to have that in my machines.

I am the same. I tested the waters a bit by getting a pocket 8086 laptop. Whilst I think it is great, the lack of floppy drive and expansion severely limits it. So I ended up buying a NuXT. I still need to actually put that in a case though.

liqmat wrote on Yesterday, 10:51:

PC Partner XL is a good option, but difficult to find. I have seen similar generic cases as well out in the wild.

I posted about the XL system here.

Nice. Yeah, that is the kind of thing I was thinking of. I can't see any for sale right now. I am guessing that they are quite desirable.

Many Bothans wrote on Yesterday, 12:01:

The high-integration slimline desktop form factor was pretty popular back in the day... An example is this CompuAdd unit - […]
Show full quote

The high-integration slimline desktop form factor was pretty popular back in the day... An example is this CompuAdd unit -

Yeah, that looks quite good in terms of height and depth. It may be the angle, but it looks rather wide though.

RetroPCCupboard wrote on Yesterday, 12:26:

Many Bothans wrote on Yesterday, 12:01:

The high-integration slimline desktop form factor was pretty popular back in the day... An example is this CompuAdd unit - […]
Show full quote

The high-integration slimline desktop form factor was pretty popular back in the day... An example is this CompuAdd unit -

Yeah, that looks quite good in terms of height and depth. It may be the angle, but it looks rather wide though.

I have a similar low profile case with a 486. It's about 14 inches wide and 4.5 inches tall.

Many Bothans wrote on Yesterday, 12:01:

The high-integration slimline desktop form factor was pretty popular back in the day... An example is this CompuAdd unit - […]
Show full quote

The high-integration slimline desktop form factor was pretty popular back in the day... An example is this CompuAdd unit -

Hi, that form factor reminds me a little bit of the Compaq Prolinea series.
Some later Deskpro and Presario models used it, too.
This page has some info: https://totalgeekdom.com/?p=356

I've had a few really small systems over the years, currently I have a "Lenovo ThinkCenter M600" and a "Zbox BI320" both super tiny, but no accessible drive bays and much newer/stronger than a 386.

The one that comes to mind for this is a small Antec "cube" I had at one point. It took a standard(small) mainboard and had two 5.25" bays (CD and floppy) as well as one 3.5"
I don't have any actual photos or recall the model, but I'm attaching a photo I found online which looks a lot like it!

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Another option you could try is to make a case. I mounted a couple systems on blocks of wood that turned out quite small:

Re: Windows 95 on a Breadboard

Note that the one of the right is much more solid now, I'm mounter the drive down and constructed a "top edge" support for the cards.
Obviously you could do whatever you want to make suitable bays.

Depending on what OS you plan to use on it, you might be able to use a "removable" USB floppy drive and CD/DVD.
Easy for optical and 3.5" - almost impossible to find for 5.25"