If I was guessing Id say previous deck height was about 38-39", and if its a true +6 (Ill spare the math here), but id add between 2' for length if you were doing a streetbike up it, but if its a dirtbike - 4ft (total length) should be fine at speed

.
One of the 77" folders (with a milkcrate) should be fine unless you are planning on beating the hell out of yourself to the point of exhaustion - then go with an 8'. The trick is getting yourself up to the deck at the same time to prevent awkwardness of a heavy XR tumbling around on you. Some people prefer ramps, some prefer step stools/crates, some buy 2wd trucks and still try to offer advice :idunno: - I've found crates or the 2 step rubbermaid stepstools to be more useful then a second ramp.
Just remember when you find a ramp you think you like and finally buy one, to go out and get well lit the following night and go to bed at ~4am. Set your alarm to 5:30 and immediately run outside and practice-load the thing, if you can do it hungover/half-wasted and asleep - it probably wont be much harder after a good day of riding.
Related Sidenote: not that I condone or advise the behaivior - but my local Sams Club has (2) flavors of loading ramps, one is 77" (single rail arched style) and the other is an 84" platform type (similar to a 3 panel straight Oxlite). They have a prett lienient return policy if you discover either of the (sub 80$) ramps dont work for ya. Both ramps are made by Dee-Zee and are capable of 500+ lbs. I own the 77" type and Ive loaded my (previous) streetbike into a 42" bed with it and no scrapes, Id think it would do a dirtbike more then easy for another 4-6" up.