3/8s front. The center of the axle doesn't matter at all. The female bolts that go into the hub and more importantly on this subject hold the peg on are 3/8s. 14mm rear almost always. Unless you have a shemale hub (3/8s bolts, 14mm center axle with nub for dropouts)
For the length of the peg i usually like a longer peg up front because it helps to get locked into smiths and anything involving the front peg. Usually like 4.5ish. i like 4 inch pegs for the rear because i dont get so worried if i mess up a wallride.
References: OneGuyIlluminatiEye, robinson79, Brian Griffin, The Horror Contact, StoreBoughtChild, C_Johnsonbmx, dkTechEthan, etc.