الأولى صح
would be

أما الثانية فخطأ

لأني ذكرت بالشرح أن if only يتبعها الزمن الماضي
and here I Were is the corrct forum of verb to be because
I am talking about conditional or hypothetical situation
which is becoming tall while I am short
unlike 'I was' ,which is about something having already happened


فتصبح
If only I weren't short


أتمنى انك فهمت , والشرح أمامك ممكن ترجعيله
حياكِ الله ^^