Help with sizing photos

This is a general question for my website. I want to upload a gallery of pictures. However, when I upload, my picture keeps getting cut off in gallery mode. When I try to upload individual pictures then they appear big and it’s hard to get them to look uniform within my blog post. Is there a way to upload a gallery of pics and fix the sizing to look uniform? Or do I have to individually size pics and pixel size before uploading them to my page? What is the largest size photos to upload to my site? I’m just a bit confused with this. Thank you. My theme is Coldbox by the way.

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