How to check Webm file dimension/resolution before upload?

Background Pony #BDA2
I accidentally uploaded a lower resolution of a webm file, because I can’t seem to figure out how to find out the file’s dimension. Does anyone know how exactly does one do that? Attempting to play it on chrome, then hovering the mouse over the tab doesn’t give a file dimension, and neither does hovering over the file in Windows Explorer, nor file properties. Googling “how to check webm file dimension” literally told me to do the above, so I’m rather confused. Please help?
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