size aics magazine for 300wm?

It is the length of the round that will fit in the magazine by CIP Europe or by SAMMI US.

Take 300 WIN MAG, it has a length of 3.340, but you may want to load VLD's that may require a length of 3.680 to fit in the magazine. So in the case of 300 WIN MAG, you would want to go with the -23 part from Accurate-Mag.

ITEM Cartridge CAP W L H MAX OAL
00100-1 .300 WM SSSF 5RD 0.736 3.715 4.01 3.615
00100-4 .300 WM SSSF 5RD 0.736 3.775 4.01 3.675
00100-23 .300 WM SSSF 5RD 0.736 3.85 4.01 3.75

One thing I have found, not many chassis or bottom metals take the longer CIP length magazines, so ask before you buy what will fit.
 
MAX OAL
Magpul PMAG 3.5
MDT 3.625
MDT 3.75

For the short action mags, Magpul is ok. But there long action mags only come in the small size.