I just had look at standard sizes from a door factory, and the widths go from 457mm to 926mm
The numbers may look very strange until you convert them to inches
457mm is 18" and 926mm is 3ft.
The biggest (front) door I have ever had was 3ft wide. I liked it a lot.
A shed chain will not stock them all, but a local door supplier (there will be one) should cheerfully order one for you after you have browsed through their catalogues shelf.
For examples, look here and click on dimensions to see the drop-down list.
Other manufacturers will be somewhat similar.
You aren't allowed to cut down fire doors much, but you can order one made to size. This is quite a common request.
Most doors can be cut down by a limited amount, this will be shown in the specification somewhere. Sadly very few are made of solid timber now, they're mostly veneered or faced with an "engineered" core which is usually scraps of wood glued together,or sometimes chipboard.
There are more trained joiners around than there are jobs for joiners. They are pretty certain to have made at least one door from scratch during their training, using just saw, chisel and plane. I had one made not long ago, he did it after work in two or three evenings. He spends his days fitting mass-produced doors and MDF skirtings.
Joinery workshops mostly cater to home renovators and use more machinery, often working to high budgets.
Late Victorian and Edwardian doors and windows will mostly have been machine-made in a works, using slow-grown softwood. Many of them are still in use after a hundred years, if they have escaped excessive damp or modernisation.
If you want a solid hardwood front door now, it will cost a lot (but maybe not as much as you can get charged for a composite door).
Don't get a hollow door, they're rubbish.