This is a goldilocks situation; you don't want them too strong, and not too weak. If they are too strong it will try to fly up once it cams over while raising and will take more force to pull it back down. Obviously if they are too weak you don't get the assist you desire.
Take a look at www.hatchlift.com
, they have selection table that shows different size beds, different mattress weights, and different hatch configurations; does the whole platform lift or just the half or so at the foot end? If you are still confused, call them during business hours, they have actual humans that answer the phone and I found them to be very helpful.
I found their prices fair and you can order kits that have all the mounting hardware and very good instructions for install.