I do not know if you are doing this for a school project, or to make a mockup only...
But that is not the way things are done in real life.
In real life, you need to find out if the project can actually be printed.
Does the paper exist on the dimensions you want? Does a printer can actually print the dimensions, is it feasible to do that size without too much waste of paper?
If you are doing a mockup only, just take the dimensions the mockup has or says. If it does not have a proper size, just make a gess. Letter? A4? A5? Just make a decision.
If it is for real, go and find a commercial printer, find out the dimensions they can actually print (print area) the paper size they can feed the machine, then you have the maximum dimensions. From there again, make some decisions on what is the best size for the project.
Buy some sheets of paper so you have a real physical dummy to feel the product, based on the characteristics of the paper, mainly weight, hardness and size.