One can achieve this using one's browser console and running
fetch('https://api.github.com/repos/[USERNAME]/[REPO]')
.then(v => v.json()).then((function(v){
console.log(v['size'] + "KB")
})
)
Let's consider a practical example.
Assuming one wants to find the size of this repo using Firefox.
Open the console with Ctrl+Shift+K.
Then paste the following code
fetch('https://api.github.com/repos/goncaloperes/TimeSeries')
.then(v => v.json()).then((function(v){
console.log(v['size'] + "KB")
})
)
Press enter and one will receive the size of the repo as one can see in the image bellow.
