Make smaller pleaseWhat part are you wanting to change exactly in that picture?
Thank you my good sirIf you search for "group.requestmembership" in HabboUI Editor you should have two options for files and they should be "habbo\habbo-1388.bin" and "habbo\742.bin". You're going to want to take a look at the 1388 one. I would assume that if you mess around with the width/height you can change the size of the box to whatever you want. It'll take some playing with to get it right but that's the file to go for.
In the future if you want to find something you can just take a look at your external_variables and external_flash_texts and search for a word from the box, so in this case "Request membership" and you will see something like this and you can then search the part BEFORE the equal sign.
Good luck with this.