An erection will only get bigger if you become more aroused. Has nothing to do with time. So if you're most aroused and turned on at the beginning of your erection, you will be the biggest then. Or, you can be fully aroused and fully erect the whole time, and it won't get any bigger. So to answer your question, the longer you stay erect does not necessarily mean the bigger the erection.
However, the biggest erection usually takes place just before and during orgasm for that session.
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erection growth? 
Linear Mode
