IRL pistol rounds (7.62x25mm, 9x19mm and 9x18mm) are about half the size of AR rounds, and full rifle rounds (7.62x54R, 7.62x51) are even slightly larger.
But currently in EFT the ammo stack system is working quite oddly as almost all ammo are 60-rounds-per-slot apart from 7.62x25 and 9x18 which are 50-round-per-slot. (Yeah I know shotgun shells are 30-per-slot but they are working fine IMO).
So, my suggestion is the ammo stack space should be reworked for Pistol Rounds (9x18mm, 7.62x25mm, 9x19mm): AR rounds (5.56mm, 5.45mm, 7.62x39mm) : Full rifle Rounds(7.62x51mm, 7.62x54R) to the ratio of 10:5:4 (i.e. 100-round : 50-round : 40-round per slot)
IMO the rework could not only encourage pistol/SMG usage by players with no access to ammo boxes/running on low-build gear by allowing them to bring more ammos, but also it might slightly help with the current hoarding-high-tier-ammo-for-profit problem as it would require more space to store AR/full rifle rounds.
actually we plan to change stack sizes. thanks for your research!