Since I can't reproduce the problem on our online demos, can you please be more specific about the configuration that causes the issue? E.g. which skin, which culture, which RadControls version, etc?
Generally, the RadCalendar control has a default width of 220px (in most skins). If you set a larger font-size or larger title format, the control may expand, which is normal. It is good to set a large-enough width, so that the control does not expand and shrink when navigating through the different months.
the Telerik team