I was on the same point a few weeks ago.

I wanted to build a bike for my 8 year old son. The Bigrock Sothea is pretty much the smallest frame you can get, but I didn´t want to spend that much money.
Therefore I ended up with a R088 in size 44.
Stem 35mm 
140mm crank
The saddle height in the picture is not final. I had to cut the seatpost and slide it in as far as possible.

Since he´s racing, the gears are limited and I went for a 1x11 Shimano 105.

Final weight including everything is 6,8kg