You can resize before posting them so they are more web friendly. I use my iPad to take quick photos for posting and I had to downsize a photo when submitting somewhere and came across an action you can create that will make a button to create smaller version of a photo you took. Apple doesn’t seem to have an option for taking smaller photos, at least nothing I could find. For web use, something in the 1000-1500 pixels on the long end should be a good size.
My Samsung photo allows me to change the resolution of the photos I take, so if that is what you are using, you can change that for photos you are posting and then change it back if you want larger photos.