Do not confuse physical sectors with logical sectors.
The physical sectors are determined by the hard disk firmware, almost certainly in this case the physical sectors are 512 bytes, until the Advanced Format (https://en.wikipedia.org/wiki/Advanced_Format) standard was created that established the physical sectors at 4kb, 512 byte sectors were used.
The logical sectors are the sector size that the file system establishes, fat/ntfs/ext, etc... for the OS to access the files, that is, the file system does a conversion between logical and physical sectors.
What Testdisk asked would surely be what size of the logical sector to use to read the data from the hard disk, I don't think it could change the size of the physical sectors in the firmware of the hard disk, and I don't think it could damage the firmware of the hard disk .
If the BIOS of a computer that supports at least 28-bit LBA (https://en.wikipedia.org/wiki/Logical_block_addressing) does not recognize the full capacity of the hard drive, it is likely that the hard drive has failed.
Another program you can try is MHDD https://hddguru.com/software/2005.10.02-MHDD/
MHDD supports detection of hard drives even if they are not configured by the bios or have been hot plugged. If MHDD does not recognize the hard disk either, it is very likely that it has failed.
I forgot to mention that the bios also performs a sector conversion between the hard drive and the file system, that's what LBA is for.
Everything explained above may contain a lot of omisions but I think it clarifies a bit how the issue of sectors works.
The fact that the HDAT2 didn't recognize the hard drive correctly isn't very encouraging, but it's important to be sure that the motherboard being used would support the full-size hard drive with no problem.