How to tell if my partner has a fully erected penis, that his achieved full erection?

When his penis is semi hard, not fully hard, it's about 5-5.5 inches, his penis isn't really stiff and hard. I've been with him one time and we didn't do much. I'm wondering if his penis is at that size when semi hard, as his penis isn't hard and I can still bend and play with it, does that... show more When his penis is semi hard, not fully hard, it's about 5-5.5 inches, his penis isn't really stiff and hard. I've been with him one time and we didn't do much. I'm wondering if his penis is at that size when semi hard, as his penis isn't hard and I can still bend and play with it, does that mean he hasn't got a fully erected penis? And will his penis be around 6.5-7 inches when hard?
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