I recently updated my version of Inkscape, and every time I opened a document, it reverted to pixels as the default unit of measurement, with an letter-sized document. I prefer using a 12x12 inch document to coincide with the Cricut mat I use most often, as well as the use of inches as my basic unit of measurement. I had changed the document template once before, but couldn't remember how, and it took me several perusals of different technical posts to finally figure it out on my own, so I am posting this to try and save someone else the trouble.
Here we go: (Inkscape should not be open at this point.)
1. First, find the drive where the Inkscape folder is located: (You can go to 'Start' then choose 'Search' to do this if you don't have a clue. Type 'Inkscape' into the Files & Folders line. A ton of Inkscape files will come up - you are looking for a folder icon. You want to find the version of Inkscape that you use - usually the most recent one. You can find out which one you use by opening Inkscape, choosing 'Help' at the top of the window, then 'About'. Don't forget to close Inkscape).
2. Save a copy of the original Default template file: Open the Inkscape folder, then open 'Share', then 'Templates'. Find the 'Default' (Scalable Vector Graphics File) template (there will be a few others that are Default_something else - ignore these.) Right click on the file, and choose 'copy'. Then left click in a blank space in the folder to deselect the Default file. Right click in the blank area and choose 'Paste' to create a copy of the original Default file. Close the window.
3. Change the Document/Template Properties: Open Inkscape. Go to 'File/Document Properties, under General', choose the default units of your choice (in my case, inches), then under 'Format', go down to 'Custom size', and choose the units you want to use, then change the width and height measurements (in my case, I chose inches and 12x12.) Close the Document Properties menu window. Next go to 'File', and choose 'Save As', and go to the drive where the Inkscape folder is saved on your computer. Open the folder, and select 'Share/Templates'. Select 'Default', and choose 'Save'. It will ask you if you want to overwrite the existing Default file. Choose 'Yes', and then close Inkscape. Reopen Inkscape and your preferred document properties should now open up each time you open Inkscape.