Navigate to the 'Contracts' heading on the BitMEX website. On the left hand side of the page, you can find a list of the contracts that we support, all the details pertaining to the contracts are listed on the respective contracts page.

Below is an example of the XBTH18 contract specifications.  

You can hover over the titles which have a dotted line underneath for an explanation as to what is meant by the term.

The contract details are also listed in summarised form on the trading page of the respective contract. You can find this on the left hand side of the page, in the 'place order' column