Hi, I have a Toyota Highlander Limited and have been told I can't tow anything longer than 17 feet is that correct I have a towing weight of 5000 lbs

The limits are normally by weight, not length. Unless there's something specific to your state.

Length had not one thing to do with towing other than the longer the object towed the easier it is to back up. Only concern you have is the towing capacity of your vehicle. If you have a trailer 5 thousand pounds and it's 30 get long you can tow it the same as one 5 thousand pounds 10 feet long. Weight is your concern not length. So whoever told you that doesn't have the first clue as to what they are saying. I have been pulling campers for 30 years i have pulled pop ups fitth wheels and currently pull a brand new keystone 34 ft with a super slide.