It generally is a crib mattress on a small bed frame. As others have mentioned, one thing to consider is whether you want it small enough to carry around (just barely big enough to cover the bed) or whether this is a quilt that can go with them to their big girl/boy bed. I make mine 65x65" for 3 reasons.
1. Even 27 x 54" is pretty long for young toddlers to successfully drag up/down the stairs -- especially if it's cotton (children up to age 3 can still struggle with regulating temperature so synthetic bedding is usually not the best option; for a lightweight batting, I'd either go with wool or QD Request cotton). Some mothers prefer a larger quilt that isn't easily dragged around the house both for safety reasons and to cut down on laundry. I make mine with a high loft cotton. It's just light enough to move to make occasional "forts" in their bedrooms, but not light enough to drag other places.
2. Realistically, I've got enough quilts to make for my 4 nieces & nephews (each 1 year apart) and clients that it's not possible for me to make another quilt 2 years from now. I want a quilt that can follow them to a Twin. 65x65" doesn't completely cover a Twin mattress, but it's close -- and certainly close enough for several more years.
3. In my family, projected heights for the children are anywhere from 5'11' for the girls to 6'7" for the boys. Since I've met a fair number of people who brought their baby/toddler quilts with them to college, I wanted to make it big enough to actually cover them as a lap quilt.