It also differs distribution from distribution. For example for Fedora minimum is 250MB[1] and 500MB is default and if you plan to (pre)upgrade in the future 500MB is required[2]. If space is not a problem I would go for 1GB to prevent shuffling partitions later as I had to do when upgrading recently.

[1] http://docs.fedoraproject.org/en-US/Fedora/16/html/Installation_Guide/s2-diskpartrecommend-x86.html
[2] http://fedoraproject.org/wiki/How_to_use_PreUpgrade#Not_enough_space_in_.2Fboot