Hi TIDALWAVE, the info Tom Ray has given you probably suits a Hobie trailer which also can be used for cats, but if you have a small boat trailer like i have and wish to add the crossbars for bolting of cradles when they arrive, check out my trailer pikkies:
My rear crossbar is about 38" long, while the front is only 18", any wider on the front would create a trip hazard when walking around.
In the next pikkie note the timber wedge i had to put under front crossbar to raise the height of cradle to ensure hull didnt scrape on trailer frame:
If you are making own trailer with own crossbars make the front crossbar 1 inch higher to compensate for rear cradle height. You might also note i still have use of dolly wheel, quite handy for moving around, my machanic mate cut down the shaft so that it could still fit and be used.