How do you know when they are too big for the crib?

June4bride2be

I fear R may outgrow his crib before he is ready for a big bed! He is pretty tall and doesn't sleep in one spot or straight. He has been falling asleep sideways and waking up feeling stuck and crying. Maybe it's just a phase, but I fear he is just getting too big for the sleeping area. He really doesn't sleep scrunched up anymore.

Obviously making it a toddler bed won't work since it is the same size, right? He can't climb out and would doubtfully stay in a regular bed. He is safer in his crib but has been sleeping poorly.

Thoughts?