I like the concept and how you implemented it. I genuinely felt pleased when I got a 0.0 result for the USB stick!
I'd suggest showing a transparent overlay of the correct size with the player's guess so we can visually see how close or how far our guess is.
I definitely think the apply is the hardest to guess because there's so much variance in real life.
Another idea for a level is guessing the size of a normal sim card or a AA battery