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.

 I have a similar problem, where on my ribbon I have a group with 3 different tools, a combo and 2 x text editors stacked ontop of eachother. 

 The padding between the tool caption and the start of the tool edit area is a constant, so therefore the edit areas of the 3 tools do not allign on the left.  On the right I could get them to allign by setting the spring property to true, so they will grow upto the border of the group.

 I tried to do the same with buttons, but although buttons have a spring property it does not seem to make them grow as big as the ribbon group..

 sorry