My ds2 is very large and not yet walking (20 months, but size of a 3 - 3 1/2 year old)
I have been offered a Maclaren Major, but have been making do with my own buggy because it does recline and therefore ds2 gets the chance to sleep when we are out and about. Unfortunately, there is no way he would sleep in an upright position
It's not particularly upright though and because it is big it is plenty high enough to support the head. It tips the child back a bit so I think a child could sleep in it though their head might roll sideways a bit rather than forwards. (I wish it was a bit more upright because Ds2 is always struggling to sit up straighter in it.)
Now for me one of the major drawbacks is that it doesn't recline (that, and the non-locking front wheels which are a pain on the bus) because ds is always going to sleep in it and lolling forwards. Easily solved though with one of those little cushions from Mothercare that velcro on to car seatbelts. I just fix one on the strap to support his chin.