Width - but crix are quite hard shelled so err on the side of smaller than larger. Tbh if they are fed in the viv together there is not much you can do to stop one being greedier than the other. What I would do is see if they will eat the crix in a separate feeding tub - one Leo at a time. This would then mean there are no crix left in the viv to bite your Leos or escape. You can also keep an eye on how much each one is eating. Then leave a bowl of mealies in the viv at all times. Make sure the food is well gut loaded and don't forget the calcium / vits.
A word of caution about keeping them together (hope I don't sound patronising - just trying to be helpful). They are too young to sex accuratey yet. Just be aware that if they turn out to be two males they will fight. If they are male and female they will breed at far too young an age. The chances are you will have to split them before too long - in fact I would split them now til you know what you have got.