As a brand-new FSSA racer, I am also in need of a lifting bridle, and by coincidence, checked into this same issue myself just a couple of days ago.
The simplest thing is to call Flying Scot at 301-334-4848 or 800-864-7208.  They will certainly have the dimensions, and my guess is that because they are just very nice people, they will probably give them to you, even though it is not entirely in their best interest, since they also sell them.  For $84, you can buy a lifting bridle from them which is guaranteed to fit, and which you know will be strong enough to do the job. For me, the peace of mind associated with that is worth the money.  I will be ordering one from them tomorrow.
If you decide to make your own, make SURE you use materials that EXCEED the expected stressloads by a significant amount.  Best of luck, whatever you decide to do.
 

Welcome to the Class !!
For your safety and the safety of others while your Scot is being hoist launched, order a bridle from Flying Scot.
Money well spent.

Unless  you're comfortable making your own seatbelts for your car, I'd order from flying scot.  The risk of damage to your boat, not to mention you or your crew, is not worth saving such a small amount of money.