Not only not resizable, you cannot collapse it.
It is being already discussed here:
http://www.kendoui.com/forums/ui/splitter/can-we-use-splitter-for-mobile-web-apps.aspx
No fix for it at the moment, if a request is made in the user voice (which apparently would be this one
http://feedback.kendoui.com/forums/127393-kendo-ui-feedback/suggestions/2335587-the-splitter-control-does-not-work-well-on-the-ipa) and it gets enough votes they will take a look on the issue.