I'm adding a new bathroom (3/4, no tub) to the upper level of the house. I've had a plumber rough in the shower supplies which are each 1/2" pex.

The thermostatic mixer has 3/4" inlets and outlets which I understand I'll need to connect to the 1/2" supplies using adapters. I'm planning to run the outlet over 3/4", up through the 3/4" volume control and up to a 3/4" elbow. The elbow's outlet will be 3/4" as that is also what the shower arm for the rain head is.

My question, before screws start turning, is whether there should be sufficient pressure to feed the head or if I need to reconsider and go for a half inch across the board (which I assume would mean a different head as well). I'm not sure if since the shower is being fed by 2x 1/2" inch supplies going into the mixer if the rest of the chain being 3/4" will work or not.

what is the GPM rating of the shower head?
are the 1/2 pex lines dedicated to the shower or do they feed the toilet and lav as well?
put a pressure gauge on a hose faucet and read the PSI standing pressure and pressure when a toilet is flushed.
why all the questions?
you may be alright but can't tell without more info..

The head is listed at 2.5 GPM.

The hot supply for the room is 1/2", with the cold supply being 3/4". The cold is branching between one 1/2" to the toilet/lav, and another 1/2" to the shower. The 1/2" hot is branching to two 1/2" supplies, one for the lav and one for the shower. The room isn't intended to have multiple people using it at the same time so it should be only one fixture running at a time.

Unfortunately I can't say on the pressure gauge as I don't have one of those handy.

why in the world did you use a 3/4 thermostatic valve, a 3/4 volume control and a 3/4 shower arm for a single 2.5 GPM shower head?
that set-up is meant for multiple body sprays and a car wash type shower head. a 3/4 inch valve is good for 15 GPM or more at 45 PSI.
what you have will be fine

Honestly? Ignorance. I found a head I liked from Moen, and their designed/recommended mixer and volume control for the head was the 3/4 mixer. The 3/4 shower arm is what was included with the head. Guess if I ever decide to update the shower to include body sprays (which I wasn't planning on at this time as I really don't understand the draw) I've got the mixer covered .

If running the 3/4 from mixer outlet up the chain through the arm shouldn't be a problem then I'll continue with the plan to keep it consistent.

as long as you are happy with the finish product, that is what matters.

Thanks very much, that helps to know the pressure should be adequate.

Now I need to come back red-faced and realize though the description and specs for the shower head say 3/4, the shower arm/head is actually 1/2 (the mixer and volume are definitely 3/4). Is it better to have the outlet from the mixer and through the volume control be 3/4 and then reduced to 1/2 at the elbow for the shower arm, or is it better practice to have the line be consistently 1/2 once it leaves the mixer and not run through reducers?

I wouldn't run 3/4 to a shower head or you will be trying to run out that much more cold water after the pipe has been sitting all night. In terms of volume, a 3/8" line would be sufficient to carry the water you need for a shower head, as the flow restrictor is even smaller than that, but 1/2" is what I would run and reduce the line at the outlet of the mixing valve.

what i usually do is use a 3/4 x 6 brass nipple between the mixer and the volume control. this (in effect) gives you one strong, solid valve that is easier to strap to some wood bracing. make sure that if you do this that the depth in the wall of both pieces works out so the trim fits the way it should. if they are the same manufacturer it should be OK. if they are different manufacturers then one may have to be slightly deeper in the wall and the nipple between will not work.
reduce the outlet of the volume control with a 1/2 inch pex by 3/4 male IPS adapter.
also use a 1/2 inch pex by 1/2 inch female drop ear elbow with 3 ears to secure it very well.