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davidcyclebackA reasonable if imperfect guide would be to look at the dimensions of uncut sheets of other cards of the period, including of non-sport cards. This should at least give you a good idea of standard sizes. I would expect there were variations and exceptions (for example, due to the number of cards in a set), but that there were a number of standard industry sizes. Sheets of cardboard and paper have always come in standard sizes, even at the wholesale and industrial level, and I'm sure the card printers bought their cardstock from someone else. Of course, the printer can cut down a sheet (they cut out the cards after all), but an odds on guess is that that a printer used a sheet of cardstock in the uncut size they bought it-- which would be one of the standard sizes.
In short, find out the standard sizes of the day, and it's most likely that the T206s sheets were standard sized. Knowing the sizes will make recreations easier. The other thing to remember is that the T206s were printed over numerous years, and sheets could have come in different sizes. Topps sheets come in different sizes.