Images are resized based on dimension not filesize. Now setting quality higher or lower will determine how big things are. These are some of the photo upload options in admin to give you an idea
Thumbnail image max width and height in pixels
Max allowable image width in pixels
Max allowable image height in pixels
Trigger medium graphic height and width
An uploaded larger than this will create a medium image of this size.
Photo uploads increment user post count?
Save and display medium and large images?
Quality setting for image resizing/thumbnails?
If photo upload width or height exceeds maximums, resize upload down to max dimensions?
Display and extract EXIF information from images?
Maximum multimedia upload size in kB
Acceptable multimedia file types?
Seperate file types by a comma and include the . (example: .mpg,.wma,.asf,.mp3)
Acceptable image file types?
Seperate file types by a comma and include the . (example: .jpg,.gif,.png)
You will still have one issue that has nothing to do with photopost though when your talking 3 mb files. Your server's PHP will be set to this by default
You will need to alter your php.ini file or if you do not have permissions ask your host to edit it to a higher number before you will be allowed to upload 3MB files.