The general method of counting in Japanese poetry is by a rhythmic unit known as the mora (morae or moras in plural). A mora is (essentially) the length of a single (full-sized) kana; so is a bit different from a syllable.
A long vowel is counted as one syllable, but two moras. e.g. えい is a single syllable, but is two moras.
ん is counted as a single mora. 関係：かんけい is two syllables, but four moras.
Yō-on take up one mora in all, because the sound is contracted into one rhythmic unit.
Sokuon count as an extra mora. e.g. 一杯：いっぱい is two syllables, but four moras い っ ぱ い
Thus, applying those rules to your examples:
かあ : two moras
きゅ : one mora
ポッキー : four moras
ん : one mora
Hope that helps!