Gelatin, when at its purest, is best measured by weight regardless of it being sheet or powder.
Sheets come in a range of "grades" graduating up from bronze to silver, gold and finally titanium or platinum. In high quality professional strength products, the difference is merely the weight of each sheet with bronze usually being around 1 gram and titanium being around 3 grams.
Generally speaking, one sachet of gelatin powder weighing 10g will set 500ml of water. Five sheets of "gold" gelatin, which is the most common of the sheet gelatin, will set the same 500ml, so long as both types are of the same high quality.