You could disable it by setting the compression quality to 100%; though I'm not sure why you'd want to disable compression since image attachments at a 70-80% compression ratio are visually quite similar to an uncompressed image at a fraction of the size.
The attachment size limit (what is allowed to be uploaded) is pre-attachment, as it is an indication of the size of files that the server will allow; then the compression is done after the file is uploaded (to save space on images that're hosted on your server).
The resulting image size is dependent on the output from the image processor (and your settings in the Control Panel); you can't really "guess" what the filesize will be.
Most of these have been answered via threads on these forums, and all settings can be found in the "Attachments" section of the v7.6.1 Control Panel.
