Modify Magento print invoice to fit different size printer

Hello I would like you to modify my Magento "print invoice" function so that the print out will fit perfectly on a 3.75" x 5" thermal label. Basically, the 'invoice' feature will now be used as a packing slip/ label. This is easier, because i won't have to generate invoices for the orders before selecting the 'print packing slip' This task will require you to modify the layout so that it will print correctly onto the labels. Currently, i am using Firefox browser and Adobe PDF reader which just resizes the text so it looks like the entire sheet but shrunk to a 3.75x5 label - make sense? When i select the label printer, it automatically resizes the label to the smaller size. Maybe you could simply adjust the layout so it prints really big text and font in A4 size page, but when AdobePDF resizes it, it fits the label? I have attached an image which shows the layout of the label, with annotations. The relevant information must fit in the necessary area shown on the label. Unfortunately i am only able to offer minimal assistance with this project since I am not technically minded with Magento, but i am with other softwares. My magento version is 1.3 Please let me know if you require clarification. I am only a small business so please be considerate with your offers, but there will be further jobs after this.