As Big G mentionned, any very short (5’2" and under) women can trigger spontaneous sizey thoughts. Even taller ones, depending on the vibe - yes, she’s hot, but what if smol
Otherwise it’s the most random things - a woman wearing an oversized piece of clothing, or struggling to reach the top shelf at the grocery store. Or just using an everyday expression (ie. “I’d like to be as small as a mouse to hear what they said in that meeting!”)
My mind wanders.