What size MDPE ?!

I am in the process of getting water into the garage for a washer.

Drainage is sorted, but now need to run a length of MDPE from the kitchen to the garage.

The foundations of the house are a raft, which rules out taking it up through the floor, atleast for me.

I plan on running the MDPE 750mm below ground level, bringing it out of the kitchen wall, lagged inside a pipe and up the garage wall again lagged within a pipe, with trace heating cable tied to it down to the bottom of the trench on both sides (See picture below).

My question is what size of MDPE should I use to reduce the change of pressure loss etc ? If that is even something to be concerned about ?

Also how best to step down from the MDPE to the usual 15mm, should this be outside within the lagging/pipe
or should i bring the MDPE through the wall with a 90bend and then step it down ?