I want to expand the master bathroom shower, but if I do I will be reducing the guest bathroom size. Is that a good idea or should I just keep it the same? Will it reduce some value, scare off potential buyers?

It depends on how the guest bathroom functionality is affected. Some houses have loads of excess space in bathrooms. That's a great candidate to shrink if you need to. Others don't, and wouldn't work.

Basically, whatever approach maximizes the functional amount of space available is the one you should take.

Good point. I need to double check on that. 

Depends on the price point and area for potential buyers.  If it is a younger family with small children, secondary bathroom size is not as imperative as a larger master bath shower.  (as long as you are not removing the tub).   If it is a small 34" insert fiberglass shower get rid of it!  

Originally posted by @Laurie Davison :

Depends on the price point and area for potential buyers.  If it is a younger family with small children, secondary bathroom size is not as imperative as a larger master bath shower.  (as long as you are not removing the tub).   If it is a small 34" insert fiberglass shower get rid of it!  

 It is a small insert and I will get rid of it. It's hideous. 

Thanks for the input guys.