What is ideal pump size for Vaillant ecoTEC Plus15kW Heat Only Boiler?

[raidac]

What is the ideal pump size for Vaillant ecoTEC Plus 400 15kW Heat Only Boiler? The property is a studio with 1 large room, a small kitchen and a small bathroom.
I've been advised to use Grundfos UPS3 15-50/65-130.

 

[Last Plumber]

Off the sound of it I would say 15/50. Not seen it obviously and the boiler isn't what I would go off.