My huge reds are legend and Dolly Parton. Neptune (true lavender)is always huge. And as a nice surprise, my Just Joey is producing very large blooms this year- and such a gorgeous color. My largest white bloom is the Pope John Paul, I am completely in love with this rose.

Medallion and Peace have large blooms.

WWII Memorial (lt. mauve/white), Fragrant Cloud (shocking orange red), Grande Dame (dk. pink), Pink Peace (med. to dk. pink), Pope John Paul II (white), Spiced Coffee (pink/yellow/tan-ish), Sugar Moon(white), Valencia (yellow/apricot) and Tahitian Sunset (orange/yellow) have the biggest hybrid tea blooms in my garden. I should add though, that these are cool weather blooms, as most shrink in our heat. All of the above are also highly fragrant. Of the lot, I would say Pope John Paul II is the most heat resistant (doesn't shrink). Most disease resistant (for my area) would be PJPII, Grande Dame, Valencia and Sugar Moon.

Legend (the Oprah rose) has very large blooms on it. It's not very good for cut flowers, but on the bush they really are huge and beautiful:)
Maude

This year, my Chryler Imperial bloom is HUGH--don't remember it being that large in the past. I mean, really, really HUGH!

Not blooming yet, but my memory is that the largest blooms in my gardens are Valencia and Peter Mayle.

Kate

The others mentioned can generate large flowers, but to answer the question in the title of you post, the largest flowered HT is Great Century. Per HMF, the "average flower size" is approximately 8". Of course, that will depend upon good culture, health of the plant, suitability to climate, etc., just as it will for all the others. Kim

Here is a link that might be useful: Great Century

Veterans' Honor, Love & Peace, Flaming Peace and Peace are the big blooms in my garden and as brittie says "the cool weather blooms are the biggest".

Do you mean the bloom size or the bush size?

Bloom size. I was looking at leagends

'Medallion'
'Just Joey'
'First Prize'

'Legends' is big for a red rose. Reds seem to be on the smaller side. Now that I consider it, the largest blooms seem to be pink or apricot, but of course YMMV.

Blue Girl can be ridiculously large.

Parole (aka, "Buxom Beauty"). Palatine usually carries it, they will open for ordering in September. The largest blooms that I grow are Hot Princess and Gemini.