Although we don't have property for the splitter size and there is no out-of-the-box way to resize the splitters you can change the theme yourself as you see fit. The RadGridResizer are used in the RadSplitContainers, RadPaneGroups and if you want to make the splitters smaller you will have to change the splitter sizes there. The different states of the splitters and panes move the resizers top left right or bottom (dock actually uses only top and left). Also when the splitter is moved the content is set a different margin which should be changed too.
I have attached a project with the RadDocking with decreased splitter sizes. I marked the places where I have made changes with "". The comments will explain what you should do so even if the current size does not suit you you can change it.
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