Hi Olivier,
Unfortunately I couldn't replicate any issue with RadComboBox DropDownWidth in Telerik version 2014. Please have a look into the sample code snippet which works fine at my end. Please provide a sample code where I can replicate the issue for further help.
ASPX:
<
telerik:RadComboBox
ID
=
"radcboDropdownWidth"
runat
=
"server"
DataSourceID
=
"SqlDataSource1"
DataTextField
=
"CountryName"
DataValueField
=
"CountryId"
DropDownWidth
=
"200"
>
</
telerik:RadComboBox
>
Thanks,
Shinu.