When our monitoring nodes perform a check on a URL, only the first 250kb is downloaded.
This means if you're using response body content checking your string must be in the first 250kb of the downloaded web page. Any content after the first 250kb will be discarded and thus content checking will not work, however this will not affect alerting on http status, code or response time.
We enforce this hard limit to ensure very large web pages do not affect the performance of our backend alerting service. Roughly 98% of web checks configured as of Dec 2015 are less than 250kb so will not be affected by this.
Contact support if you're unsure how this limitation will affect any of your service checks.