After a long time of researching on the web, found the solution on one of IBM's pages:
http://publib.boulder.ibm.com/infocenter/dmndhelp/v6rxmx/index.jsp?topic=/com.ibm.btools.help.monitor.doc/Doc/tasks/administration/ad_settingthequeuedestinationsize.html
Copying it here in case the link becomes inactive in the future...
1.From the WebSphere Application Server administrative console, go to Service integration > Buses.
2.In the Name column, click busName, where the busName is the name of the bus where your queue is defined.
3.Click the Destinations.
4.Click destinationName.
5.Click Queue points.
6.Click queuePointName (for example Monitor_Bus_Queue_Destination@node_name.server_name- MONITOR.cell_name.Bus).
7.In the High message threshold
field, type the value of the maximum queue size you want to use. The default value is 50000 messages.
8.Click OK.
9.In the Message information box, click Save to apply your changes. The Save page is displayed.
10.Click Save.
Note this is for WebSphere 6 but it will work the same in later versions as well.