You want to set up your own often used vessels in your logbook so that when you add an entry into your logbook you simply select the vessel by name. This means you're not adding length and tonnage etc each time you make an entry.
We have set up some vessels already for you but you should definitely set up your own. Below we have added the boat called "Boat Drinks" (named after a Jimmy Buffet song for those parrot heads). Under the Vessels link, click on the Add button and follow the instructions there. For tonnage - just as a reference, a 40 foot monhull is about 15 gross tons. You can upload a picture of your vessel. Next time you are out on the boat, snap a pic using your mobile phone and use the NauticEd App to edit your vessel.
Use the Common preset up Vessels for when you just randomly went out on a boat you'll probably never go out on again.
Different length vessels apply to your certifications. For example you must have experience on a 28 ft (8.3m) boat in order to gain the Bareboat Charter Master Rank. So it is reasonably important that you accurately set up the vessel.