Start with the dry weight of your boat. Add fuel, water and your best guess on the weight of any upgrades (Extra battery, A/C, hot water, upgraded motor/outdrive over base, anchor etc.,). Next add your best guess for the "stuff" you will take with you when you go boating(food, towels, ice, outboard for dinghy, dinghy, etc.). The whole point is to get your best estimate of your boat loaded down and ready to go. When you have that estimate add a cushion of 1,000 lbs or so and that is the weight your trailer should be able to handle. Your boat DRY weight is available through the specs on the Sea Ray website.
The trailer will have a VIN # and capacity plate from the manufacturer. That capacity should be greater than your estimate above allowing sufficient reserve capacity. Assuming it's adjustable, you should be able to move the bunks/rollers to adjust it to your boat. Also check to make sure it's long enough to handle the 260 you are considering. You can measure from the bow stop to the end of the bunks and compare that to your 260. Check to see how much the bunks can move if you need to adjust them fore and aft as well as side to side. Some bow stops are adjustable and can be moved forward or backwards to get the tongue loading right as well.