30x30cm makes a nice sized trifold. Don't forget to cut it a little bigger to allow for seam allowance. Depending on the age of bub you can make it a bit longer for the rise too so for a large it can be 35 - 40x30cm.

Bamboo itself isn't as drying to the skin like microfibre is but as it is an absorbant fabric it will be wet against the skin which may cause a rash. Microfleece is a nice barrier type liner that isn't absorbant. Problem is it's a little hard to come by this time of year as it's a "winter" fabric
So you don't get them mixed up microfibre is like a short pile towelling like the ones sold to clean with and microfleece is a fine polar fleece type fabric that you can make baby blankets from.