45 cm SVT in arm cause

About a month ago I started feeling a weird pinprick in my right arm just above the elbow. I brushed it off but about a week ago I developed a couple of painful, hard lumps in my forearm and the vein was hard and ropey. Last night I decided to go to the walk-in clinic and it turns out to be a 45 cm SVT, the “longest the doctor had ever seen”, which is 1 cm from the subclavicular. I’m to take Eliquis for the next month to keep it from breaking loose.

I had a blood draw in that arm a few days before the initial pain started, but it wasn’t anything out of the ordinary. Could that have caused the huge clot?

I’m also concerned about how long I may need to be on the blood thinner. Does the size of the clot make any difference?