A question about water preassure.

I am new to this forum and I hope someone can answer my question. First of all I would like to say hello, and I am very happy to have found this forum and am sure I will learn alot from all of you.

My question is about the water preassure in my shower, I have noticed a gradual loss of preassure. The preassure appears to be fine every where else in the house, but both the hot and cold water is weak coming out of the shower head. My tub spout also holds the diverter switch and it is broken so it is always stuck to feed the shower.

I replaced the shower head and when that didn't solve the issue I removed the tub spout, the preasure from the spout seems o.k. but there is a loss coming from the shower spout.

The pipes feeding the valve assembly seems small to me also.

My question is what size pipes should feed the valves and how can I boost the preassure to the shower head?

Thanks in Advance,
John