I'm going to say anything from 5" - 6.5" is "average" length, anything above that is big. Girth I'm going to say anything from 4"-5" is average, anything above that is big. I'm mainly going on statistics and having actually looked into this outside LSPG (this site got me interested in trying to find the truth of the matter, as it seems like an un-answered question).
Now, I'm fairly sure this is a controversial opinion, so I'd like to argue for it.
First, a point I'd like to put forwards is that LPSG is a lovely site (one of my favorites!) but there is a reality disruption field when it comes to the size of male genitals (balls included, hah!). The truth of the matter is that people here (not all) often have very skewed perceptions.
To those who put 8-plus in a "medium" category, my opinion is that you've really shifted into some sort of self-justified fantasy land. Find me a scientifically attested 14" penis, or even a reliably attested measurement. I've taken a visual forensics unit (yay for uni electives!) and really, when it gets to 14" up against your body, you're talking about a penis that reliably reaches past your own sternum when you're sitting down.
Instead, have a look at the following graph:
Imageenis percentile.svg - Wikipedia, the free encyclopedia
The study wasn't perfect, that's for sure... but if anything it's on the high side. Yet, it shows that if you have a 7" penis, you're already statistically the biggest in a group of 20 men! Even 6.5" and you're still the biggest in a group of 10. A lot of guys here who get called big are actually 6.5" or 7" (even though they say they're much bigger). This becomes an internalized lie, people rely on these numbers so much they come to believe them.
A lot of people listen to their partners when it comes to size; the 1 in 10 6.5" becomes an 8", the 1 in 20 7" becomes a 9" then as well. Search on an adult internet dating site by penis size and you'll find that it's what every guy is saying he's packing. Look how many times on this board you see an actual larger guy having his number inflated hugely by a partner.
Another way to put it in perspective, Peter North's well attested size is 8.5 x 6. Are these guys seriously saying that Peter North's penis is medium sized?
If you actually look at scientifically done medical studies, the numbers are lower than the lifestyles by far, *especially* in girth, where a lot of men are thought to self-select themselves from studies. Some urology studies actually put the average girth *below* 4" for an erect penis. Makes you feel lucky, doesn't it?
If anyone wants a better reality check, some of Madame Zora's (and others) posts into the statistics on penis size will really put things in perspective.