One of Roald Dahl's finest...

Oh, you're one of those guys who takes all the best spots and camps out for sunrise... bet you never thought about everyone else waiting for their chance to get a photo. Smh my head.

Mine are pretty much the same - never caused any trouble, still there some decades later so not everyone needs them extracted...

Apparently worth downvoting though shrug

Uh... Ceaușescu was the communist leader of Romania, seemed pretty obvious to me...

Username checks out...

Taking a practical engineer's approach - what accuracy do you actually need? You may find the difference between the actual length and the measured length after stretching and increased spool circumference is largely irrelevant. Or you can apply a conversion factor after the event to get 'close enough'.

Alternativel, an idea out of left field... could you find a way to precisely weigh the spool and calculate the mass of added thread, assuming you know the weight of a reference length of the thread to then convert it to a length?

Do you actually mean Styrofoam (dense, closed-cell foam, often pink, blue or black) or expanded polystyrene (white squishy beads pressed together)? The latter stuff is widely and incorrectly called Styrofoam, but the two things are quite different and advice for one doesn't always work for the other.

Expanded polystyrene (EPS) is virtually impossible to cut really cleanly without a hot blade or wire - the beads will tear out and the edges will snap.

Actual Styrofoam (XPS) can be cut nicely with knives etc. but if you have confused the two materials this could explain why you are having trouble.