min. space for toilet

i have an older home and was wondering what is the min. space required for the toilet drain to back wall. i have about 11 to 11- 1/2 inches on center to finished sheetrock

This distance is what is referred to as rough-in. The standard is 12" from the center of the drain (flange) to the finished wall. In the real world, there are many variances, so yours is not too far from the standard. Toilets come in 3 rough-in sizes, 10", 12", and 14", but realize that these distances are do not mean the toilet will fit tightly against the back wall even if the flanges is exactly the prescribed distance. There will always be space allowance, and different brands and even different models in the same brand will have different degrees of space. In your case, a 12" rough-in toilet will fit just fine if the actual distance is 11-1/2", but if it is actually 11", you may want check what actual space is required. There are some other ways to match a toilet to a space that is a bit too tight, so don't panic. The Toto line of toilets has several models that use their "Unifit" adapters that can fit a toilet to any of the three rough-in sizes. Besides fitting the spaces, they are outstanding performers and are very attractive units as well.

Many 12" rough toilets will have space behind them. I believe the Cadet 3 fits there.
The TOTO Drake can be installed at 10-7/8"
The Drake II at 11.5"