Pictures show too small and text too large in posts

When I insert a picture in a post it is very small – about 300 pixels. When I try to enlarge them they get really blurry even though the quality of the picture is excellent. I think this has something to do with the fact that the theme automatically compresses the images upon upload. I’ve gone under settings —-> media and entered the max pixels as 1024×1024 for large size photos but I think it’s defaulting to the medium size of 300×300 and I don’t see any way to select if I want to use medium vs large. Also the text is very big and I can’t figure out how to adjust that either.