Montessori floor mattress: crib vs twin size?

Definitely get a twin size mattress! Your kid could be on it for a long time (mine was!) and as they age you get to lie down next to them to read them stories, among other things. We got a futon and it was amazing. My daughter would "go down" for naptime in her room, and she'd crawl around a little bit, look at a book or a toy and eventually nod off. (Yes, we peeked under the door to see what she was doing.) My friends were astonished that she didn't struggle to go to sleep. The futon on the floor was the best things we ever did in terms of parenting hacks. She went from bassinet to the futon. Good luck!

Our little one has been on a crib sized floor bed (same as the one you posted a link to) since she outgrew her HALO bassinet (so about 3-4 months). We put a bumper on the side so she doesn't roll out (although now she is 1 and can crawl over it). It's worked out great! We went with the crib size because it was easier to find a mattress that we knew was infant safe. That being said, if I could go back, I would probably go twin sized because it would be so much easier to lay with her on the mattress for snuggles or reading. 

The thing about a floor bed for a (soon to be) crawler is that they may end up awake in their room, without you, for quite a substantial amount of time. you have to think about safety vis a vis absolutely everything in their room: are the socket plugs failproof? Are there any lead paint chips peeling or floating in a crevice near the baseboards? Can any toys in their room, especially wooden ones, injure them if you’re not there? (My kids went through a bashing-all-toys-against-forehead phase). Worst of all, what happens if you or a partner or a pet end up leaving their door open overnight? No house is fully childproof for an unattended crawling 8 months old. Frankly I’d get a used crib (maybe even on your local Buy Nothing) until they’re at least 18 months, a tiny bit more sensible and past the phase of putting absolutely everything in their mouth.

We moved our oldest into a twin-size floor bed (with a sprout frame) around a year old and our second made the transition around 10 months. I recommend a twin size for a couple reasons - first, they can keep the mattress as they get older! Second, it’s nice to be able to lie with them if needed. Definitely make sure you baby proof the room that they’re in since they’ll have free access.   We’ve loved the floor bed as an alternative to the crib, especially since neither of our kids were big fans of the crib. 

We had a crib for our first kid but also disliked it - it just took up too much space. Our second and third kids were in a Pack'n'Play until they were ~1 year old, then we moved them to a mattress on the floor. You might want to consider a Pack-'n'Play - it's inexpensive, easy to move or store, and good to have on hand for traveling anyway. I didn't feel ready to put them on a real bed (or mattress on the floor) until they were about 10-12 months old - I still worried about getting suffocated by a pillow or blanket.

Once we moved them to a mattress, we put a large (6x6) baby fence around to keep them from some non-baby-proofed items nearby, so that is also an option. Or you can baby-proof the entire room and close the door so that they're safely contained.

I've use both mattress sizes and would get a twin mattress over a crib-sized one because (1) they're eventually going to grow into it anyway, (2) if you end up needing to cosleep with them, you can fit, and (3) even if you don't cosleep, it gives you room to sit on it, for example to read books. Otherwise, it's uncomfortable reading bedtime stories with a floor bed.

I recommend the twin size. Then you can use it when they're older, too. If you put it on the floor it's only like a four inch fall if the child was to roll off during the night - we had our mattress directly on the floor next to a rug for my son starting around 6 months and never had issues with it.  This was the mattress we chose, and he still uses it at age 5. https://www.naturepedic.com/2-in-1-organic-kids-mattress-buy

Mine slept in a pack n play (with mesh) and nap tent (kidco peapod) after aging out of the bassinet. No crib. Neither felt prison-like, while keeping baby safe. Still using these at 18mo and no bed on the horizon due to baby finding it very cosy (I suspect it’s the nest/den-like form factor). 

Hi! I love that you are considering skipping a crib. I actually do sleep and floor bed parent education and didn't use a crib for either of my children. I recommend a twin size mattress, extra firm, 4-6 inches thick. I moved both my babies to a mattress around 3-4 months old. I'm happy to answer any questions you have! Feel free to reach out.