The best way to go about this is to simply do the simple math and add up the numbers. You can get the specs online. To the weight of the boat, add the engine/engines, full fuel/water/head, any options over std, batteries and add at least 500lbs for gear/ice/etc. Add another 10% for good measure and that's the weight capacity you want. Length is determined by measuring the boat - typically you measure a straight line from the bow eye to the garboard drain plug (or the furthest aft area of the hull that the bunks/rollers would rest against).
Some good trailers that you'll find in your neck of the woods are Shoreland'r and Tidewater. Load Rite is OK, too. Stay away from Venture.
Now... the easiest way? Let the shop that you are buying the trailer from do the work for you. After all, you're paying them!
Are you doing the proper setup on the trailer or is the shop?