That really depends on the code requirements for your part of the country. Here, the vent must be a minimum of half the size of the drain it serves so 1-1/2" vent would be the minimum size for a 3" drain. However, in reading through the posts since I've joined, I've seen that some municipalities require a 2" vent for a toilet. Call your local building dept. and ask what the requirements for your area are. I'm sure they'll be happy to help you out.
On a side note, I'm installing a bathroom in my basement at the present time. My vent for my toilet will be "oversized" to 2". I just like to "overdo" now and then especially on my own plumbing.