Are steel doors available in standard or custom sizes?

Both. Every door and partition has a default size on its product page, for example 900 x 2100 mm for the single-pane door, and is also welded to measure for your opening, so you never adapt the opening to a catalogue format. For a door made to measure, the quotation states the width and height of the opening, the hinge side and the direction of opening, and the division of the frame into panes, before anything is cut. If you are not sure how to measure, send a photo with the dimensions you have.