Assuming you are talking about a combi boiler - unless you live in a 20 bed mansion the size of the boiler only matters in terms of hot water flow. The bigger the boiler generally the better the hot water. If you are talking about a system boiler then you do not want to go too big.
I have always been a Vaillant fan and steered clear of Worcester because of bad experiences from 10-15 years back. However, I recently had a Worcester 12i fitted (I was talked into it by the fitter who is a mate) and I must say I am 100% impressed. It is the quietest boiler I've ever come across and heated our 20 year old 4 bed bog-standard house though the cold spell with plenty to spare.
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