The amount of memory available on the interactive nodes is described on the Interactive (login) Nodes page. The interactive nodes are shared among all users and running high memory jobs on the interactive nodes slows them down for everyone. If you think your work might be disruptive you should either run it in batch or run it interactively on a batch node as described here.
The batch system knows how much memory each node has and how much memory each job requires. By default each job is given 1.1GB, but you can request more or less memory by following the instructions in Submitting Jobs. However, it should be noted that the more memory you require, the harder it will be for the batch system to schedule your job, especially when the cluster is filled to capacity. Therefore it is in your best interest not to ask for more memory than you need.