Indeed, when you get the size of the panes on the server to save them they will always be in pixels and if you set the size in pixels, the splitter and the panes will not resize correctly when the browser is resized which is expected.
What I can suggest is the following:
1) Save the dimensions as they are in pixels
2) When you need to reset the size, calculate the percentages value based on the saved dimensions of the splitter and the panes and set the corresponding percentages
In this manner the proportions should be kept and the splitter will be sized in percentages so that it will resize along with the browser.
All the best,
the Telerik team
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