When setting a course, I hear some people say its 'gonna be windy' so bring the targets in a bit. Lack of wind the opposite. Just shot a Winter League course the other day when the targets were shortish as it was windy, but as the shoot started the wind dropped and loads of high scores. So its difficult to suggest anything with guaranteed results.
If the aim is to make the shots harder, then I think 15mm targets out as far as is legal. The slightest wind and you miss them.
At our club we regularly have the odd couple of targets at 58 to 62 yards and this makes the course a challenge in a no wind condition. If its windy its not so bad as not many people hit them. If you do, however, its a big bonus. These are just internal shoots so the rules are sometimes bent a little.
Personally I would like to see a long lane added to all shoots just like an extra discipline.
EV2 Mk3+Sightron SIII 10-50 x 60 LRIRMOA / Bushnell 8-32
Theoben Rapid 12 FAC
GROWING OLD IS NOT FOR SISSIES