It's meant to be fun, so shooting at overly small targets makes it a frustrating game of chance.
The equipment your using will have a bearing on a sensible target size as do the weather conditions. A 40mm spinner at 75 yards on a calm day can be relatively easy, but with even a light swirling wind it can become a lottery.
Try shooting some groups on paper, then use a target which is 1.5x the size of your best groups.
If you are using a knokdown target or spinner then simply adjust the range depending on the equipment and conditions. It's handy to have a backstop which shows your fall of shot if you miss, a piece of cardboard from a cereal packet for example, you'll learn more and be less frustrated if you know where the misses are going.
Have fun.