It depends how long the pipes are, and what flow you're looking for. Ie where on the pump curve you want to be using it.
You have to weigh up how much the pipe resistance is going to matter.
For the bath, 22mm pipe would be better than 15mm, your bath would fill a bit quicker, but it's usually less difference than you might think.
For the shower,
If you're feeding a 12" diameter head with big holes and little resistance, then you need to keep the pipe resistance particularly low.
With a more restrictive ( smaller holes, needle jets) head, then the pipe's resistance will be a small fraction of the head's so it matters much less.
And if all the flow's going to be sharing a 1.5metre shower hose, then that'll be more resistance than quite a lot of the (separate) supply pipe(s) even if they're only 15m. So eg for a couple of metres it wouldn't make much difference what size they were, for a middling head.
Bigger pipes mean more "dead" water, so more cold before the hot arrives, otherwise no detriments.
