Measurement Units for Width and Height

I am working on a project which the width and height of should be really specific.
With that i am not sure on which value should i set the width and height because i do not know what units of measurement they are using.
for example if i put the width of 100 . What is the unit of that 100 ?
Like is it : 100px, 100mm or which unit ?

I tried to find the answer in the documentations , but was not available !
Wish i can get the answer here :)

Thanks,

No file or test is named in the issue. Start by locating the width and height options in the documentation and implementation, determine which measurement units they use, and update the documentation so the unit is explicit with an example.

Written by the indexing model from the issue text.