Apologies in advance if the title of my question is unclear. Please edit if you can think of a better way to phrase it!
From time to time, you need to explain how a foreign name is spelled in katakana. Example: The first name Bart is commonly written as バート.
Imagine that you are applying for a credit card on the telephone. What is the correct way to explain this katakana spelling?
Example: 名{な}(ファーストネーム)はバートです。バ、(?)、ト。
How to explain that the first katanana character (バ) has a long vowel?
Related topic: How to explain a small vowel? Example: Tio might be written as ティオ. How to explain the small イ? Normally, I would say テ、小{ちい}さいイ、オ, but I'm not sure if this is correct.