minimum space you need for a toilet is 15" from the center of the flange, so min 30" wide (and 12" from the rear wall if you want a 12" rough-in toilet.) If you had more room, i would allot a few more inches on either side for more breathing room. me, being a bigger guy, it makes it difficult to use a smaller space. think of resale, you never know who will buy your house. the front of the toilet needs to have 24" min. clearance in front of it. This is all from me researching half baths a few years back. i forget what sites i used, but someone will come along and suggest some more info, i'm sure.
look under "small half bath plans" or small half bath ideas" and use "images" and you can find what you are looking for.