If you know how big the space of a banner (blocks) is in Minecraft, you can also build and render the images in the corresponding image size beforehand: 1 block ≙ 128px.

This allows you to create a tab-completion for the width arguments, which displays a value based on the file size. In the simplest case, only if the file is divisible to 128 without remainder (n mod 128 = 0). And if the width has been defined, the height can also be calculated and output as a tab-completion.

Especially if you still have to work with the flags, you need all the arguments so that the syntax is correct, right?