Actually, the small 'tsu' is a consonant doubler (it indicates a geminate consonant) not a vowel doubler, so if there was a consonant for it to double, I'd agree (as it does look pretty small). When doubling vowel sounds, a matching vowel kana (in this case "a" to go with "ba") would be used (and was used, in this case) or since the sound effect is in katakana, the Katakana-Hiragana Prolonged Sound Mark (it looks like a dash mark) might be used (it's rarely used in hiragana).