Does this refer to the actual physical size of the card? So a ticket would be the present credit-card size unless there was more information on it than could reasonably be printed in that space? I can see advantages in that (it's simpler to have one piece of card per journey) and disadvantages (the present size fits nicely in a wallet or pocket). And as an aside, why do we have this size? I have a vague memory of tickets being longer, though perhaps not as long as the tickets used currently in (some? all?) other parts of Europe (the ones that are sort of pink-blue-purple patterned).
Yes, the current ticket size constrains what information can be printed. There is a new design on the credit card sized stock but the content looks too squashed. The new tickets can be as long or short as needed to fit the relevant information and they can be folded easily, to fit into wallets or purses.
The larger tickets were issued by a few online retailers many years ago. They are still issued by some travel agents and sales agents overseas (they can use the same ticket stock as they do for domestic tickets within their own country). I seem to recall seing someone with a BritRail pass issued like this in the very recent past.