In my experience, the mitigating factor to weaving width is unused heddles at the edge of each harness. You should be able to weave 21.5” easily, with just a little crowding at the edges. At the reed there will be no crowding, so weaving full width should be fine.
There are weavers who weave panels and join them together to make larger projects. Some people favour decorative joins (feature the join), others go for as invisible a join as possible. The choice is yours. Sampling before commiting to a full sized project is a good idea, if only to ensure the length of the pieces is easy to keep the same, how to join the pieces, and whether you like the look. I know, sample, sample, sample! But for a large project, it can save time, money, and frustration.