Not only is it too short to accommodate that boat, the weight capacity of the trailer may not be sufficient to carry that much weight.
For example, a normal 16ft boat will only weigh so much, so 1000lb springs (2 per axle, so 2000lb capacity) is usually sufficient. But jump up to an 18ft boat and the axle and springs will need to carry more weight, so they are often 1500lb springs, again 2 per axle, which also needs to be rated for 3K, so 3000lb capacity.
Apparently, because you want to make the trailer 3ft longer, you might be jumping up to a 19ft+ boat on a 16ft boat's trailer. Probably weighs nearer to 3000lbs and might NEED 1750lb springs (2per to carry 3500lbs) w an appropriately sized axle. Whats the weight of the boat and the trailer as is (weighed separately)?
Boat trailer makers dont use more spring then is required for the load a trailer is designed to carry. A 1500lb boat on a trailer rated for 3500lb will get beat half to death because it isn't heavy enough to utilize the trailers suspension, whatever type it has. And heavier suspension components cost more, so that makes the trailer more expensive then it really needs to be.