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In RMVX my text goes off the screen and I can't read half of the things I get my characters to say. I tried puting it in full screen and nothing happened.
If you want to make the text box wider, you can edit one of the default scripts (I think Window_Message, but I'm not sure). Ask a scripter how you do it. Or you can get a custom message system that has the height and width function built in (like ccoa's UMS).
Lol a little tip when typing in the box, if you have a faceset along with the message, type only to the first downwards triangle floating above the textbox, if you don't have a faceset along with it, you can type all the way to the second downwards triangle floating above the textbox.