I wish there was a way to have the size of the Genesis levels, though.
There's a good chance I can make that happen by hacking the game, but there are still other subtle game mechanics differences that needs to be addressed if you want an exact emulation of Genesis Lemmings.
A completely different possibility is the reverse, to write a program that allows you to import levels created in Lemmix (with suitable restrictions of course such as size) into the Genesis game ROM.
Don't really have time nor interest for any of this though, sorry.