It depends on your aspect ratio. For normal screens this is either approximately 4:3 (Standard TV or monitor) or 16:9 (Widescreen). There's slight variations on this like 16:10 as well, but the two above are a good guide.
If you're creating a side-scroller, I'd imagine it's the left-right that matters more than the top-bottom. I'd suggest a widescreen format, so around the 420-430 mark will do for height.
In terms of small screen resolutions: if you're happy with 760 then you don't really need to go any lower than 420. Since most monitors will have at worst this 16:9 resolution, if it can fit 760 across then it can fit 420 down. Even a really old resolution like 800x600 can fit it comfortably.
If you're talking about supporting phones or people making their Facebook window take up half the monitor rather than the full amount, then I'd check out this tutorial on resizing your game dynamically. You want to keep your aspect ratio constant and resize the whole thing to make the game as big as it can be.