It's not really a workspace memory limit - it's the Eclipse application's memory limit. Eclipse is a Java app - ignoring the binary Eclipse from Red Hat, which I doubt you're using. So you can set JVM memory specifications when you start Eclipse just like you would for any other Java app.
If you get too many large-scale projects in a single workspace, you might need to break them out into multiple workspaces, since not only will you be consuming large amounts of memory, but also Eclipse will be spending a lot of time keeping them all properly managed. However, Eclipse can handle a lot, so unless you're already running more than about 2GB of VM RAM, that's probably not necessary.
An IDE is no substitute for an Intelligent Developer.
If you are working on windows, you can set the heap size for eclipse itself using
eclipse.exe -vmargs -Xmx768m
(The example sets teh maximum heap size to 768 MB.)
I guess it works similar with other operating systems.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus