I reference this spreadsheet in a some of my Youtube videos on how to use MS Publisher to create clipart. www.youtube.com/c/rachelportercba.
The Excel spreadsheet is easy to use. You need to know the size of at least one balloon in the drawing or real life piece and the size you want it to convert to. An example of me using the spreadsheet in scaling a balloon wall can be found here - https://youtu.be/XM9adYQB1qk?t=5m50s