Trying to replace my door. Size guide help? ?

When you buy a new door, you buy it with a new door frame.  You remove the trim from one side, cut the nails holding the frame in place and take out the frame with the door latched in it.  Then you put in the new frame with the new door in place, get everything straight and lined up, shim up the frame and nail it in place. Put the trim back one side.  The trim on the side you leave in place is sort of a guide to help you with the new frame.  Now, it requires some strength a 2 person job makes it easier.  You should have at least an inch of width around the door that you shim up.  You may need some small boards as well as shims.    Now somewhere in all of that you have to paint or stain the door.  That will require taking the frame off of the door (note) in order to coat them properly.  Finishing before installing makes things a lot easier.  I read your comment and my comment is you do not know what you are getting yourself into.  That edge of the door is beveled and you are talking about hand beveling a door edge and keeping it neat. Those carpenter skills no longer exist.  You will ruin your expensive door, and then go buy one in a frame, or call  a door installer to put in the door.  But you should do it because you are going to get stuck with painting/staining it, and you want to do that first.

how about measuring it with a tapemeasure !! it does help you know ...

Unless you are a skilled carpenter fitting a new door into an existing frame is very difficult. Getting the hinges in exactly the right place is very difficult. Go with a pre-hung door. There are several videos on Youtube that show how to install a new pre-hung door.

Look on the door sides and top for a manufacturer's name and model. Then find a retailer who carries that brand, and see if they have a 'slab' door that will fit and a style you like.

This way, the hinges will already be mitered and the door latch & dead bolt will drilled out, so it will plug & play.

OR get a pre-hung one, as already suggested. 

Princess Pounder is right. A new door and frame will be a much better prospect. They are energy efficient, draught proofed, secure and need no real maintenance. Buy a white one as this will reflect the heat you mentioned.

You will likely not find a perfect fit replacement. I strongly suggest replacing the entire door AND frame. its easier that way. And ensures a perfect seal and fit. They are sold as prehung doors. Thats how its done these days.

Rub cake bath soap on the joints to make drawers slide easily, old trick. Doing that, I'll bet you a dime against a donut your outside door moves, closes or opens.

It will fit ....Get yourself a pre-hung 3/0 door. 

Follow the guide lines given by the previous post. Once you remove the trim you will see the rough opening which will be approx 38 x 82 1/2.

Doors are made to fit the " rough in"  size. That is the distance between the studs in the wall the door is mounted in. Take the trim off the door and you can see the studs and shims that  are present and then measure the space between the studs to get the rough in size.