In the state of Virginia, is it illegal for a minor to buy a pocket knife, and are there size restrictions?

Specific to dirks, switchblades and bowies:

§ 18.2-309. Furnishing certain weapons to minors; penalty.

A. If any person sells, barters, gives or furnishes, or causes to be sold, bartered, given or furnished, to any minor a dirk, switchblade knife or bowie knife, having good cause to believe him to be a minor, such person shall be guilty of a Class 1 misdemeanor.

http://leg1.state.va.us/cgi-bin/legp504.exe?000+cod+18.2-309

There's no specific statewide law restricting purchase of a legal knife by minors, but apparently vendors won't sell to those under 18 or sometimes 16, depending on the vendor. A minor can buy a knife online without restriction, other than they must use a credit card. It's not illegal for a minor to possess a knife.
As for size restrictions, there doesn't seem to be any.
http://thefiringline.com/library/blades/knifelaws.html
Switchblades are illegal, among others
http://leg1.state.va.us/000/cod/18.2-308.HTM