Have you looked in your Sentra owners manual for towing limitations? You could also call a Nissan dealer for more info.
I looked up your Owners manual on line, on page 9-16 under trailer towing, it simply says "Do not tow a trailer with your vehicle."
BUT with that said, I think almost all motorcycles also say to not tow a trailer, but many of us do with no ill effects.
You are on the right track to want to keep things as light as possible. You may want to consider having electric brakes on the trailer. Often times it is safe braking capacity that is the biggest limiting factor on trailer weight.
A 4x8 trailer will carry the bike, BUT you will not have room to move the bike forward or back to adjust tongue weight.
A friend of mine got a 4x10 all aluminum trailer from a local welder that also makes custom trailers, that is very light weight and has the room to move the bike around for proper balance.