boiler size ufh underfloor heating

Hi, In the middle of an house extension and would appreciate some guidance on boiler sizing. The house is a 3 bed detached two storey house built in 1960s so has small 50mm filled cavity walls. Width = 7m, depth = 8m, double glazed & insulated, room height = 2.4m. Extending two storey 3.2m...