On the FIP website the different recommended exhibit sheet sizes are shown.
Normally you may put 16 A4 pages – 4 rows of 4 pages – in one frame. These pages can be 2 cm wider (not 21, but 23 cm) – like album pages. The broader pages may cause some problems in frames of certain countries, so that you have to overlap pages and not all the texts are visible.
You may also use the A3 format, so only 8 pages in one frame (or 4 rows of 2 pages). According to the recommendations on FIP website these can be wider (from 42 to 46 cm). The same problem here with broader pages in frames of certain countries.
An alternative is the so-called “Damian Läge” format, because he was the first one we have seen to use the format in a thematic philately exhibit (and many followed him already). In stead of 4 (A4) or 4 (A3) pages in one row, you make almost square pages of 29 cm. With this format you have no problems to show some broader items, typically for thematic philately exhibits: postal stationery envelopes (f.i. the French PAPs), telegrams, original artwork, etc.
In the example shown on the FIP website height of these pages may even go to 31 cm, but this could give problems in some frames of certain countries that only allow a height of 29,7 cm. If you have higher pages, you may receive your pages back all damaged above.