Welcome to the Lilypond Sandbox! Please enter the incipit as if it were injected into the \score block of the lilypond input file (as demonstrated above). Use this page to ensure that your incipit looks as intended and does not take up too much space. As for the size, kindly keep your length of incipit to around one measure, or approximately ten to twelve notes in a row. Please note that incipits are optional, so do not feel pressured to enter them unless you know what you are doing. Consider the above examples for some inspiration for the length of the incipit.
If you would like to learn how to enter music in lilypond and create incipits, feel free to read the quickstart quide at lilypond.org.