Vikti,
You are probably going to get as many different answers to your question as there are responses. You will surely not get a consensus because everyone has their favorite. For me that would be a 1/4" or 3/8" straight blade and I can give you all kinds of reasons why, but that still only makes it the best blade for me. I have seen Bill Gomer take his favorite blade, which is a 1/4" angle ceramic, and do some of the most awesome work anyone could hope to do. I once watched an old saddlemaker use a 1/2" home made knife and do incredibly fine, detailed work with it. Bob Brown, who did work for such Hollywood stars as Roy Rogers and John Wayne, used a sharpened screwdriver to carve with and could make a very compelling argument as to why that was best.
The real secret to doing better decorative cuts is not in the swivel knife blade, but in the hand holding it. The reason those mentioned above were and are so accomplished is because of the many hours they have spent perfecting their skill. Choose the knife and blade that you like the best and then put some miles on it. If someone had kept track of how many miles of swivel knife cuts some of these people had put in leather, you would know why their decorative cuts are so beautiful.
In other words, practice, practice, practice...
Jim L.