First of all, thank you for your interest in UI for NativeScript.
I reviewed the reported issue and indeed at the moment this is a real limitation and there is no possible way to specify the width of the input fields.
For your convenience, I logged new feature request here
Unfortunately, I could not commit to the exact timeframe, when this functionality will be available. Regarding that, you could keep track on the issue for further info
Let me know, whether this is applicable for you.
Telerik by Progress