There's currently no good way to do this. The toolbars manager is designed to mimic the Office and VS style toolbars, which base tool size on content. I would recommend submitting a feature request for this: http://devcenter.infragistics.com/Protected/RequestFeature.aspx.
You can created a derived ButtonTool and override CalculateLogicalSizeOfToolInstance. When your override gets called, call the base implementation to measure what that tool should be, then you can go up to the tool's owner and iterate through all of its other tools. If any tools are also an instance of your derived class, you could call the base implementation of their CalculateLogicalSizeOfToolInstance. Use the largest width of all those tools in the size returned from your overload. The only problem I see with this approach is it can be very time consuming, and will get exponentially slower as more of your tools are added to a container.