The dimensions for toolpost tool holders are published in catalogs, so the lathe user can compare the dimensions to the lathe dimensions.
If one chooses a QCTP series/size that's too large for a particular lathe, then the setup may not accept suitably sized cutting tools for that lathe.
A small QCTP will have an advantage of allowing the user to use larger cutting tool sizes, in some cases.
Taking the TOC top of compound (plinth) distance to the lathe spindle centerline height will show which particular QCTP series/size will be best suited to that lathe.
The important dimension is the height of the tool holder's shelf/cutout for the cutting tool to rest on. This dimension determines how large the cutting tools can be, by adding the desired cutting tool height to that dimension, before the top of the cutting tool can no longer be adjusted down to the spindle's centerline.
All the common dovetailed QCTPs allow the holders to slide to the bottom, stopping on the TOC.
If one chooses a B or 200 series QCTP for a small lathe, the holders may prevent 3/8" cutting tools from meeting the spindle centerline (tool tip too high).. since the holder may come to rest on the TOC. Then the user would need to use smaller cutting tools (possibly 5/16 or 1/4"). Choosing a toolpost series that's too small could prevent the cutting tool tip from reaching the spindle centerline (without adding a riser block).
The primary consideration, IMO, is scale.
Choosing a B/200 series for a 9" lathe doesn't guarantee more rigidity. One wouldn't expect to run 1/2" or 3/4" cutting tools on a 9" lathe, and they likely wouldn't fit due to the limited spindle centerline height over the TOC. Considering other factors such as motor size and the lathes's overall rigidity, should determine which size cutting tools are appropriate for a specific size of lathe.
Choosing a B/200 series for a 12" lathe is generally more reasonable. Using an A/100 series on a 12" lathe may be completely satisfactory and perform just as well, depending upon the usual chip loads and type of workpieces.
Reviewing the dimensions of tool holders will be the determining which holders are going to be suitable for future purchases, but also for the initial QCTP choice.
Some other factors might be which sizes, and how much existing tooling one already has in the shop, and perhaps a less common need to use some unusual sized tooling such as a large boring bar that was only $5 at a garage sale.