This site has some answers that might help you: https://www.hunker.com/13402164/dimensions-of-standard-hearths
Here are some ideas as well to look at in addition to Holly's! Good luck.
It depends on the fireplace size. The link above is good. I have a gas fireplace, so I don't have a hearth. For a wood burning fireplace, the width has to be wider than the firebox and deep enough to catch sparks.
Consult the Building Regulations for restrictions on proximity of combustible materials