I am afraid that the desired resizing functionality cannot be achieved for the ImageManager dialog. The main reason for this is that the control used for listing the files and folders into the dialog is RadFileExplorer and it does not support size in percentages (the pointed in your image resize handler is part of this control).
The only possible workaround I could suggest you is to enable the resize behaviors of the ImageManager's wrapping RadWindow:
sender, EventArgs e)
DialogDefinition imagemanagerDefinition = RadEditor1.GetDialogDefinition(
imagemanagerDefinition.Behaviors = WindowBehaviors.Resize | WindowBehaviors.Move | WindowBehaviors.Close;
Note, though, that this will not make the ImageManager's inner controls recalculate their size and you will experience many other visual glitches.
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