Thought I would post on this as all of the forums I could find on the topic were really old. I have just returned from a bouldering trip to Font for which I used the Eurostar there and back. I was packing a Snap Bun Bbq pad which measures 100 x 60 x 35cm when closed which in theory was about 15cm longer than what Eurostar permit you to take on. At both ends the staff didn’t bat an eyelid, the only difficulty was getting it through the Xray machine on the Uk side where it actually got stuck for a moment! On the French side there are much larger bag scanners. Whether they would kick up a fuss if one brought on a much larger pad I am not sure, the only other observation is the luggage racks are quite small on the train so you may have to ask them to point out a spot that can store it.
As an aside…once you are at Paris Gard du Nord the quickest way down to the Font area is the direct train from Gare du Lyon to Melun, it only takes about 20 Mins - from there it was only 20 mins in a cab to Barbizon and you can access the whole of Cuvier and Apremont bouldering areas on foot from there so no car needed.
Getting around on the metro with a bouldering pad is a bit faffy but otherwise it was an easy way to get down!
