Well there's no real constant for how much space an average script will take up. Depending on the length of text and what you want your event to do it can vary quite a lot. For example, the following script takes up 15 bytes:
Ten of those bytes are taken up by the three commands and their parameters while the other five bytes are accounted for by the five letters used in the @talk pointer. Even extending the text by a few words can double or triple its length so there is no way you can talk about an 'average script length'. If you look at each command in XSE command guide it will tell you how many bytes each command will take so if you really want to count their lengths, just add up each command's size and then the length of any text you've added.
Scripts take up very little space in the ROM (they're absolutely tiny when compared to graphics) so there's no need to be concerned about how much free space you need, particularly if you use a dynamic offset like I have done.
edit: my math is dumb i suck