Anyone who picks anything smaller than 3x3 needs to have his/her head examined. Because there is a limited number of rewards, largest per square reward performance is irrelevant; you should get the largest reward buildings, which offer the largest total boost to your pop and culture compared to the standard buildings.
Um, yeah. No. Welcome to reality, where things have to be taken with context in consideration. As you don't appear to care for the politically-correct phrasing of things, I'll roll with your wishes and try to help you see how big of an idiot you come across as by making this claim.
First off, you're not 100% wrong. There is a fallacy in jumping all over something because it offers the best per square value. But amusingly enough, it is on par with the fallacy of jumping all over something because it offers the biggest gross numbers. (Implying that you, sir snowman, might consider cutting in the head-check line that you're forming for those who disagree with you.)
Consider this: if Inno offers a new value reward "UNICORN'S HORN", a 1x1 with a ch.1 value of 75 culture and also say they also come up with a new largest building "GIGANTIHUGE'S KEEP" a 5x7 with a ch.1 value of 900 culture. {Both these numbers are pulled from current top-tier rewards x 1.2, so if squinted at in just the right poor lighting, we can excuse them as being fair, or perhaps equally unfair.}
You have cleverly discovered that per-square value isn't the whole of the story, this holiday most people are likely to get around 10 buildings, meaning that someone going all in for 1x1s will get a total of 750 culture, while someone going for size will pull in a whopping 9000. Stack on top of that your astute observation that neighborly help is less than efficient with smaller buildings, the poor newbie with ten 1x1s and two friend visits has just barely caught up to the culture of one of the 5x7s, whereas those same two visits would bring in a 1800 more culture for the colossal connoisseur.
But the poor sap who swung the other way is ever bit as screwed, if not moreso. As the proud owner of ten 5x7s it will now take you 14 expansion slots just to lay them all down. By the time you have the room to spare, you'll have progressed to the point where other holidays have happened, or simple progress has opened up higher value buildings, rendering the holiday prizes wasted in your bags. Even the ones that you do place will age quickly, and will be some of the first buildings to be scrapped. A 1x1 that has been outgrown and no longer is the best value per square will still find a corner or road-end to brighten up for quite some time. A large building is quickly trashed and the space used for better things.
So, wait. It seems like (gasp!) blanket statements of "x is best, and anyone who chooses y needs their heads examined" might be (the horror!) ill-thought out‽ What‽‽‽!!1!!one!!!
Adaptability is key. If you have tonnes of space, go ahead, lean harder on the larger buildings. I personally am hoping for 3 of the Phoenix Cults, they'll work just fine for my planned layout. But small buildings have their place, in fact, lots of them, and new places can and often will be found for them. The Rainbow Flower Cage is awesome; having 3-4 of them is like having a free residence that gives culture instead of taking it, doesn't require a road, and can be broken apart and tucked into literally any possible opening. They're so good, it generally takes either a very tight new optimization run or another holiday offering Rainbow Flower Cages before you'll ever recycle them. And things in the middle, are in the middle: easier and faster to place than large things, more impactful and buff-friendly than the small things. Usually a variety is ideal, with at least one or two larger, sub-optimal buildings catching neighbor buffs, a few small things tucked in the awkward corners that can't be otherwise filled, and the rest spent filling what space you have now and in the next 2-3 months ahead with as high of a value-per-square that still manages to fill your allocated space.