On the national exhibition Phila-France in Mâcon I have checked all the thematic philately exhibits regarding sheet sizes.
So more than half of the exhibitors still only use A4 pages. If they choose the larger pages, in France it is till common to use the A3 pages (and not the size in between A4 and A3 as is seen now o,n international level more in some German exhibits). The A3 is not easy to handle and to fill the page correctly. Some of the exhibitors still divide a lot of these A3 pages in two parts, sometimes even separated by a line in the middle of the page (example 1 below). But some others are able to fill the whole page and take care of the fact that you have to read from left to right and from above to below.
If you have a large subject and you want to use A3 size, this may cause you problems in putting the subtitles on the pages. With A3 you only have half of the pages, so you only can use half of the subtitles that you are able to use on the A4 size pages. With a narrow theme you may not have this problem.
Another problem with larger pages maybe the transport of the exhibit. This may make it difficult for the commissioner to take your exhibit with him (sometimes by hand luggage). If you only have very few A3 pages, it has already happened that people who are dismounting the exhibits on some international exhibition and are not aware of what they have in their hands, they want to fold these big pages so they have the same size as the rest of the exhibit – A4.
An exhibit on “playing cards” used the size of a big A4 playing card with rounded edges (see example below at the right).