If they really do stretch it, then it means you have to leave them some area to stretch. Eg give them an inch to stretch into two inches. But if you don't know exactly how much to allow, then all your questions about sizing and sharpening are completely moot.
DON'T trust them with this important task. Do it yourself.
Great! So what's the problem?? Just close your logo PSD file, and never open it again.
When you need to use it on a photo, go to File>Place, exactly as my instructions say.
First things first, please make sure you always do your raw processing properly. And I urge you to consider The Raw Class, it will change your life.
To help you with this further, I need to know what screen you have, and which calibrator you have.