Fix length of input field

Dear experts,

I would like to create input field which is only accept 5 characters input by users.

But I don't the exactly data type I should use for field declaration in my custom business object.

Or should I do it in UI Designer ? 

However I still did not see any options to set MAX LENGTH of input field in UI Designer .

How can I achieve  this ?

Especially in extensibility mode with extension field .

Best Regards,

Krit R.