Size of a rest

For what reason? Select the rest and tick "small" inside the inspector?

Normally, sizes of rests are pretty standardized by the rules of music engraving, and you shouldn't be going out of your way to make them different - unless you are also making the notes smaller, like to notate a cue part or a cadenza. In that case, the Inspector is indeed what you want. If you have something else in mind, best to attach your score and explain in more detail what the special circumstances are so we can understand and advise better.