I've searched the forums and it looks like 4mm (5/32) is the standard vacuum hose ID... However, the thickness of the hose itself varies from hose to hose... silicone is also supposedly the best material for vacuum lines.

I went to autozone and they didn't have 5/32 in stock, neither did Lowes, so I ended up going to Kragen (O'Reilly Auto Parts) and buying a box of THERMOID 5/32 vacuum line. It was 3 bucks for like 2-3 feet. The outside is somewhat thick and it doesn't seem like it would expand much... is this bad?