Tell me what you know about toddler beds

[katymf]

DS is 18mo. I've noticed that sometimes when he is sleeping and i go into his room to pick up toys or just to check on him, he looks like he is too big for his crib. his arms going through the side rails, when he rolls over he immediately hits the side of the crib. he is a big boy to begin with, very long. he just looks uncomfortable. every night he wakes up after an hour or so of being put into his crib and cries and i really believe it's because he doesn't have enough room in his crib to sleep comfortably.
he ends up in bed with us every night which is fine, but we ourselves have a full size bed and i am 29 weeks pregnant so it gets a little crammed.
my question (or questions) to you all is:
1. how old was your LO when you transitioned into toddler bed?
2. when you get a toddler bed do you still use the crib mattress or switch to a twin?
3. from photos i've seen online, it looks like a toddler bed with both side rails looks just like a crib. is it really any roomier?
4. from my story, do you think he would need something bigger then a toddler bed? twin size perhaps with just one side rail? the bed would be up against a wall so he wouldn't roll off the other side.
thanks for the help!