The PC must has a specification of it, how many inches is the screen.

You can look up the model number with a search engine. Screen size is often given as the diagonal measurement 

One of the most accurate ways to determine the diagonal size of a device is to use special programs for this purpose. For example, by installing the AIDA64 utility on your computer (there is a free trial version), you can scan the device and get all the technical specifications.

I use a ruler, or a tape measure. Normally, they round up the actual screen size to the next whole number.