-vmargs -Xms[###]M -Xmx[###]M
where [###] is replaced by heap size in megabytes.
These values can be also edited in eclipse.ini configuration file located in the same folder as wid.exe where WID shortcut points at.
In our experience 768 MB is enough for initial heap size and 1536 MB for maximum heap size for the most memory expensive operations such is working with WebSphere Business Services Fabric Toolkit.
You can monitor heap size and start garbage collection at any time with heap monitor. In WID, from the Window menu select Preferences / General and check the Show heap status checkbox and click OK. You will get heap monitor as shown in the image below: