There's also steps in post #1, to expand system partition...
If the gsi ROM you're trying to flash doesn't fit on current system partition, then,
*This process will wipe your system partition and will reset your recovery, so, keep the dynapatch zip and gsi image on hand, you will need it!*
Download latest platform tools from AOSP site, link below
Android SDK Platform-Tools is a component for the Android SDK.
a) download the product.img from above link
b)Reboot to recovery from magisk app, then, in twrp, click on 'Reboot' button
c) click 'fastboot'
d) the device will now reboot and then after a few seconds, a twrp ui will appear with the text 'fastboot'
e) connect your device to a pc with the latest version of platform tools and drivers installed
f) run these commands in order
fastboot flash product product.img
fastboot erase system
fastboot resize-logical-partition system 3940000000
fastboot reboot recovery
g) after doing so, again reboot to recovery, flash dynapatch zip first
h) again reboot to recovery and flash your gsi image
i) this time images up to 3.9GB will be flashable
j) no need to format data if you're flashing same rom
And that's it you'll be able to enjoy bigger ROMs on your Galaxy Tab A7!