The smallest I found during a quick surf is 34" from Gravely (probably also available from Ariens). Both Hustler and BobCat have one in 36". Probably most manufacturers figure if you need something smaller, you'll opt for a walk-behind.

Here is a link that might be useful: Gravely ZT 34

Dixon, now owned by Husqvarna, DID make a 30" for years. Don't know if it is still available or not.

Walt Conner

"Dixon, now owned by Husqvarna, DID make a 30" for years. Don't know if it is still available or not.
Walt Conner"

Actually, they still do. I didn't look there due to their affiliation with Husky; I figured the models would be the same.

Disclaimer: Keep in mind that I am not making any claims here regarding the durability of any of these homeowner series ZTRs.

Here is a link that might be useful: Dixon SpeedZTR