A quick Google produced the following information from the Wisconsin Department of Transportation...
Trailers
Does a homemade trailer need to be inspected?
No. Homemade trailers need to meet the requirements for lights and chains. If over 3,000 pounds, an ID number must be assigned by DMV.
What are the requirements for agriculture bailer combination?
A farmer may pull two trailers behind a tractor (hauling corn, etc.). Pulling with a truck requires that both trailers be empty. Motor trucks may tow two new trailers or semi-trailers to be used for transporting farm products or livestock without a special permit if:
1. The trailers are being delivered from dealer to dealer or manufacturer to dealer.
2. Motor truck is no more than 25 feet long.
3. Each trailer is no more than 28 feet, 8 inches long. (Frame extension isn't included in determining the length.)
What is required to operate a three vehicle train-- car/camper/boat?
See Multiple trip permits. The owner must obtain a permit from DMV and carry it while towing the extra vehicle. It is not legal to tow two trailers or other combinations of two or more vehicles.
How much does this permit cost?
Fees range from $27 for any three month period up to $40 for a full year. Permits are valid on all highways in the state.
I'm pulling a trailer, does it need lights?
Yes. RE: Wisconsin statutes 347.13, 347.14, 347.15.
When does a trailer need a slow moving vehicle sign?
When the trailer is traveling at a speed less than 25 miles per hour. RE: Wisconsin statute 347.245 (1)
How do I license my trailer?
If it is more than 3,000 pounds it is required. Under 3,000 pounds is optional. See Registration, miscellaneous requirements.(Which says... Homemade trailers:
Title and registration are not required in Wisconsin for homemade trailers less than 3,000 lbs. gross vehicle weight. However, if you want to register a homemade trailer, complete form MV1 PDF (401 KB) indicating the weight at which you wish to register the trailer. Attach information on the type of trailer, width, length, axles, year built and the builder. Enclose any receipts/bills of sale you have for the parts used to build the trailer. DMV will issue a Vehicle Identification Number (VIN). You then need to permanently attach this VIN to the trailer and have it inspected by either a licensed motor vehicle dealer or police officer. After you have returned the inspection form, DMV will issue a title and registration.)
I just bought a trailer which is not licensed. Can I pull it home?
Yes. If it does require a license, register with a motor vehicle inspector.
When does a trailer need trailer brakes?
Trailer brakes are required if the trailer and load exceed 3,000 pounds or if the trailer's gross weight exceeds the empty weight of the towing vehicle. RE: Wisconsin statute 348.10 (4)
How big can my trailer be before it needs side mirrors?
You must be able to see 200 feet of the roadway to the rear of your vehicle. Re: Wisconsin statute 347.40
Does my trailer need safety chains?
Yes, unless it is a fifth-wheel and king pin assembly mount. Other fifth wheel mounts require safety chains.
I'm towing another vehicle. What rules and provisions do I need to follow?
You need directional and stop lamps (tail lights if required for water conditions or night driving) to the furthermost rear. You may not use a chain longer than 12-feet. You need a flashing yellow light in the rear if you are traveling under the normal speed. Re: Wisconsin statute 347.26(6)
I'm hauling a long piece of material (i.e., telephone pole). How many feet may it stick out beyond my vehicle, and may I operate on a main highway while transporting it?
Three-foot overhang on front of vehicle: four-foot overhang on back. If longer, you need a red flag (12-inch square). A single vehicle cannot be more than 40-feet-long (truck and overhang load). Must have a red light visible from 500 feet at end of load if hauling during hours of darkness. Two vehicles and load may not exceed 65 feet.
Also...
http://www.legis.state.wi.us/rsb/code/trans/trans308.pdf gives some more informaton that pertains to trailers.