Definitely don't want the hole to be overly large in a public setting, but you also want to make certain the hole is large enough to be safe for just about any size dick. Sharp or splintery edges ain't fun for the top, nor, for that matter, the chin of the sucker.
By the way, if you're building one at home make sure to include one little extra: something for guys to grab hold of while they use the hole. They'll love you for it.
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