Generally, in order to a have correct, safe, and fun-creating slope, the length of a straight playground slide is approximately double the height of the deck to which it is attached (but this will vary somewhat).
Example: An 8' long slide is intended for use on a deck whose surface is about 4' (or 48") above the play surface, 10' for a 5' deck, 12' for a 6' deck, 14' for a 7' deck, et cetera.