What is the wheel base of a tandem tractor with a day cab?

There isn't a set length for this. If you have a COE tractor in daycab configuration, you could be looking at 100 inches or less. If you have, say, a Peterbilt 379 with the sleeper cut off, you're still going to have a 265 inch wheelbase.

- - - - -

Within reason, you can get any wheelbase you want because new trucks are custom made. If you've bought a new car in the last ten years, you know all cars are made the same (except for color) and you just buy one off the lot, kinda like buying a pair of shoes. There's no such thing as dealer stock in new trucks. If you want one it takes about a day to order it, and one of the things they make you specify is wheelbase.

The next question is why. Depends on what you haul. Farmers are the big malefactors here. They need short trailers so they can get in and out of their farms, but they also need a lot of weight capacity. If you have a 38-foot grain trailer and you need to pull legal maximum, you've got to move the kingpin on the trailer back. This increases the amount of front swing on the trailer, and to maintain enough distance between the back of the cab and the nose of the trailer that you don't knock the cab off the frame, you've got to increase the wheelbase. And you can't think, "oh, they'll never catch me overweight because there's no weigh station between me and the grain elevator" - I live in wheat country, and the cops around here have portable scales. Every year at harvest time, they pull those scales out of the warehouse and start making money.