How easy must it be to find a website's terms of service? I assume that if it's very difficult to find, then users couldn't be reasonably expected to know the terms of service. Are there any guidelines for where the link to the terms of service must be, what font size it must use, et cetera?
Generally, terms of service must be prominently displayed.
Click-wrapped terms have been found enforceable if the user is required to view them prior to engaging in activities on the website.
However, browser-wrapped terms have been found unenforceable if a reasonable user would not be expected to view the terms prior to engaging in a transaction.
As an example, when you register an account on the Stack Exchange network, your viewing of legal links, and continuning to use the site after this, is recorded. This could be used as evidence that you agreed to the terms.
There are views that the link to the terms should be placed in the upper-left quadrant of the homepage - this will mean the defendant would need to prove that they ignored the link.
As for font size, while there are no general requirements per se, it is necessary that they be legible (that is, not excessively small).