No, the bigger the better. A larger expansion vessel will reduce how much the pressure changes as expansion happens.
Note this is for the hot water, not the central heating, I think. This is not a combi boiler but a boiler with a water tank. Spent half a day trying to figure out the plumbing pipe work and gave up as the pipes disappear under floors and in ceilings.
It really doesn't make any difference what the expansion vessel is connected to. It's there to keep the pressure reasonably constant as the water heats up and cools down, so a larger one will cause less pressure change. The only reasons not to go for a larger one is cost and space, along with the fact that some pressure change is acceptable, so if you know that 5 litres is adequate, 12 litre is just more money and a bigger thing to hide. However, if 5 litres is marginal and could lead the pressure changing too much, or you have a 12 litre expansion vessel available, you might as well use it.
As both need similar fixing arrangements, and either can be fitted by one person, there's little to chose between them. If you had a 1 litre or smaller vessel, it might be possible to mount it on a pipe, saving the cost of mounting on brackets etc. If you had a 100 litre tank, it would need serious mounting hardware, but neither of those extremes apply here.
