In usual handwriting, kana often turn out smaller than full-fledged kanji, especially kanji with many strokes. (Though practising writing both kanji and kana the same size is probably an important step towards achieving a nice balance between kanji and kana size.)
Some fonts have comparatively small katakana, which I find very easy to read.
This is from the book 時をかける少女, 角川文庫
I don't think half-width katakana have anything to do with handwriting. Half-width kana are squashed in the horizontal direction by a factor of two and used for saving space in typed documents (e.g. on 通帳, in ATMs, etc.), when choosing a small enough font size would be too small to read easily. I don't really recall seeing half-width kana mixed with kanji.
In any case, I think it's fine to write kana a little smaller than kanji, but I don't think you should aim for squashing your kana horizontally.