You know this issue very well. It's been years that the Merchant services window is microscopic and Intuit has done nothing to fix it. 

It is not compatible with 4K monitors.  This is despicable to say the least. Everyone has 4K monitors and why would I change the resolution to a decade back so I can see the merchant services letter?   

It's allover the NET! google it.  Millions of users with the same problem you have not addressed.