I feel like this is a really stupid question but I'm at my wits end. It's been so long since I've used a real camera (not my iPhone) and I'm trying to take a decent quality pic so I can blow it up to poster size. My digital camera has the capacity to make pics be up to 7M (I'm assuming that's 7MB?). In the image size option, there's 7M, 5M, 3:2, etc. I made sure to choose 7M. But when I take the pic and upload it to my computer, it's only coming out approx. 2.75-3 MB. I'm NOT zooming in when I take the pic (I'm assuming that'll reduce the file size?). What am I doing wrong that it won't use a bigger file size? Thanks!
No, it's not 7MB on the camera, it's more likely measured in 7MP - megapixels.
My phone camera takes 16MP photos, which are around 6.4MB, so your file size seems correct.
Remember also that most digital cameras will save the image at 72ppi - pixels per inch. For good photo printing that will need to be 300ppi, certainly no less than 150-200ppi (if printing at home) or the image will look grainy.