1. First of all you need a rooted android as Xposed installer can only be installed on a rooted android, so Root your android to proceed.
2. After rooting your android device you have to install the Xposed installer.
3. Now after having xposed installer on your device, you need a cool Xposed module that is Gmail App Attachment Size the app that will be increasing the maximum attachment size limit in your Gmail app.
4. After downloading this module install in your device and then enable it in your xposed installer by simply clicking on the tick on the Xposed installer app.
5. Now you simply need to reboot your android device to let this app works perfectly on your device.
6. Thats it you are done, after reboot the size of Gmail maximum attachment limit is automatically increased and you can check it by sending out some large files than 5 Mb as those will got sent without any limit issue.