How to reduce the size of a door opening to fit smaller door?
If you cannot find an adequate door to cut down your only option is to build out the frame to accommodate a smaller door.
Once you select the door that works for you build out the framing on top and sides using whatever sized material that will get you to the proper dimensions. 1x4 or 1x6 or even 2x. The casing can be installed over it and trimmed out shimming where necessary..
You may have to use extra wide trim around the casing to get the look you want. Since it's a fire door be sure to check your local codes concerning the type of door required and compliance requirements regarding the framing and method of closure.
It would be a lot easier if you can find a door that you can trim down or order one custom made..