For Windows 98 gaming, I'd definitely go with a CRT whenever possible. But as to what size, that would depend on the games targeted.
Generally speaking, for gaming at 800x600 or lower resolutions (especially DOS stuff @ 640x480 and lower), I think a late 15" will more than suffice (and still probably sync as "high" as 1024x768 for desktop, which should be perfectly adequate for Windows 98 use.) A 17" will be fine too, but just won't be needed.
Past 800x600 (i.e. 1024x768) but not higher than 1152x864, I recommend going with a 17".
For 1152x864 to about 1440x1050 (I don't and won't ever recommend 1280x1024 on a CRT - it really looks ugly, IMO), jump up to a 19".
Past that (e.g. 1600x1200) go for a 21"
*BUT* here's a trade-off I've noticed with quite a few CRTs: 15" and 17" tend to have (or can be adjusted to) amazing contrast levels: pitch black that makes the monitor look like it's completely turned off in a dark room and blinding whites when transitioning from a dark picture. This can also be obtained with some 19" CRTs. But jumping up to 21" CRTs, I've found many of them can't quite achieve the same levels of contrast. It often becomes a trade-off between either bright whites (with "washed out" blacks) or deep dark blacks (with rather "murky" whites), but never both at the same time, especially on Sony CRTs. IDK, maybe I'm just not lucky, but both of my 21" Sony CRTs are this way. A 21" Hitachi I have is a lot better, but still not perfect. In contrast (pun intended 😁 ), all of my 19" CRTs seemed to suffer from this a lot less or not at all. And with my 17" CRTs, it's almost never a problem (except, again, with a few Sony's I encountered.)
All of that said, I'm not a complete CRT "purist" and will game on LCDs too. But every time I jump on one of my CRTs, I'm quickly reminded of why the look so much better in old games: built-in anti-aliasing from the "imperfect" pixels of the CRT, no motion blurring regardless of the FPS, and much less noticeable screen tearing (especially in games with "jumpy" or slow FPS.)