Could you please confirm which version of RadUpload you are using? You have posted in the the "RadUpload for ASP.NET Ajax" forum but you seem to be using RadUpload "Classic". The easiest way to tell is to check the assembly name. If it is RadUpload.Net2.dll you are using RadUpload "Classic". In this case you can check this
help topic. In case you are using RadUpload for ASP.NET Ajax (from the Telerik.Web.UI assembly) you can use the InputSize property.
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