I think the size of show pens is fine for the average breed of cat , if in doubt you can always request a double pen when you send in the show entry but its not cheap, i think some of the larger breeds like main coons or adult ragdolls can need a double pen, As for cats sleeping in their litter trays, lol, i don't have any idea why a lot of them do this , mine do it too but as long as they are not stressed i leave them too it , and as long as the tray hasnt been used of course. a lot of cats love to trash their show pens too,
best wishes...........Chris