My husband has recently started playing golf, and has had several people tell him that his clubs are too short for him (He's about 6'). Can clubs be resized, or do they have to be bought longer? I have been looking online at new clubs, and they don't offer sizing information.

Also, how does one know what the right length of club is?

Go to a golf shop and have them take measurements and tell him the ideal golf club length. You don't have to buy the customs clubs they will undoubtedly offer but at least you'll know the right size.

Also, no you can't re-size clubs to make them longer, shorter might be possible but not longer.

Yes,. you can lengthen golf clubs, called butt end extensions. However I am doubting why at 6 feet your husband need longer clubs unless he has very short arm. Depending on your club now, it may not be worth having them extended, depends on price and quality.
Golf Professional-theebump.