Photoshop resolution

How can i made sure something is saved in the highest resolution?
I Edited a picture in Photoshop cutting a few pictures and merging them together and when i upload the picture to snapfish or any site to print in large it says low resolution.
How can i save it that i shouldn’t have that issue and it should stay clear?

When you save you can choose the resolution.

What is the size of your image?

Thank you!
I was able to do it in light room.