I feel pretty silly because I was messing around with the settings and changed the UI scale to 3 (which is a lot bigger than my phone screen actually supports : http://imgur.com/Y4B21gh), and thus the slider is now so big that I can't move it back to 1.8 I was using...

Does anyone have a clue how I can get it back to the way it was ?

Try landscape mode to create more space

I feel so silly right now... Thank you ever so much !