Fonts are scalable vectors — the size is set in the application that uses them.
Your design should fit entirely within WinAscent and WinDescent.
The symbols in my own fonts align with the baseline and top of capitals so that they work well with text. Symbol fonts without text don’t have to follow the same rule.
100 pix will do, but is a bit small for importing fine details — 500 pix would be better. Scan at a higher resolution, or export from a drawing application as AI or PDF, and import those into FontCreator.
See this thread (among others) for a discussion of font size.