You can control the size of the minimized items with the MinimizedColumnWidth and MinimizedRowHeight properties. By default value of the MinimizedColumnWidth is Auto which means that the width of the minimized items will depend on their content - the problem that you're experiencing, so you should set it to something like 200 or 250 and everything should be ok.
About the second question - the MinimizedHeight property of the items is taken into account only
when the MinimizedRowHeight property is Auto (else the height of the minimized items comes from its value, not from the MinimizedHeight).
I've attached a sample project that demonstrates this so could you please examine it and if you have more questions feel free to ask?
the Telerik team
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