OK I know it sounds confusing so here I go. Doing some work on the washer supply lines. Well they are copper right now 1/2". Issue is there is leaking around the shutoff valves and such (old fixtures). So the question is it OK to go from 1/2 to 3/4 in piping? The distance would be about 6" for each hot and cold pipe and then and elbow then 4" to the new shutoff valve if that. Shark bite connectors for the valve and what i would be getting is 1/2 to 3/4 couplers for copper to CPVC.
I don't think the shutoff valves leaking should have anything to do with the diameter of pipe you're using. It sounds like the situation is: you are replacing that last bit of pipe anyway because the shutoff valves are leaking, and you want to know if you can also increase the pipe diameter while doing this.
Changing the pipe diameter will not do anything to fix leaky shutoff valves, and it probably won't have any benefit, but I also don't think it will do any harm.
If it is less than a foot of travel, it's hard to see how flow could be impacted much, unless it is getting constricted from something huge down to a half-inch. Keep in mind that the flow will be restricted by the smallest diameter pipe. If a 1/2" line is what's coming out of the wall and you couple it up to 3/4" for the last foot, you're not really changing much.
If the shutoff valves are leaking repair or replace them. Why go to 3/4" copper? It sounds like a fairly simple repair. Sharkbites are fine but I don't see why you would need them.
It's okay to go to 3/4" and use sharkbites - won't hurt a thing if installed properly. Maintaining the 1/2" throughout would have been the cleaner way to do it. With pictures we might have been able to help you simplify what you're doing. Simpler is always better. But seems like you're trying to work with the material you have. Go for it but simplify where you can.
I like solid pipe to my valves. A larger diameter pipe is stronger , but a properly anchored valve will be fine in any size. Will it hurt to go larger NO. Is it a waste of $ ? Probably. Replacing the valves would be my option but I have the proper tools , if you don’t have the tools for copper a shark bite will be fine.