You or your webmaster is the best judge of the size of your site. (Spiderline cannot know how many pages are on your site without physically crawling it.) You can purchase a lower plan than needed and then upgrade if you hit your service limit. Credit for all the unused days of the lower plan is immediately applied to the new one. There are also many factors that might play into how many pages are eventually crawled, like eliminating certain paths that don't need indexing.