I want to use this plugin to make a large sword within the game, but it seems that no matter how I do it he will only have the size of a diamond sword, is there any way to achieve the effect I want?

What is this correspondence called in the editor? I did not find the relevant options, and also custom models support 1.12.2?

Yes, you need to use a custom item model for this. In particular, you should increase the scale.

When editing a custom item, you should be able to choose the Model below the Texture. You need to select a JSON file, which you should write yourself. See https://minecraft.fandom.com/wiki/Model#Item_models for documentation on how to create it.

Closing this due to inactivity. Feel free to reopen this when needed.