3" Fowler from MJ Engineering is a good size, Ouite a few about and a good network of builders if you have any queries. The odd casting can be on a bit of a long lead but most are OK
Or if you want a road loco the 3" Mac now done by HPM is a good size and the 4" even bigger. The Little Samson in 4" is a quicker build but I don't like the look myself.
And final choice is a 6" Gold Medal Tractor which comes out about teh same size as a large 4", infact it uses the sam eboiler as the 4" Mac
J