No bigger than a flea

When you want to describe the size of something unfamiliar by comparing it to a familiar object, there seems to be an iconic list that changes very slowly. It does change, though. For example, objects about the size of a pack of cigarettes are now more likely to be compared to a deck of cards, and nothing is compared to the size of an ocean liner any more.

Hence:

Size of a pea.
Size of a grape.
Size of a fist.
Size of a baseball.
Size of a grapefruit.
Big as a Volkswagen Beetle.
Big as a school bus.
Length of my thumb.
Length of a football field.
Tall as the Empire State Building.

More iconically-sized things, please.

Large as life.

Bigger than a bread box.

As much as X swimming pools
an area the size of Wales (why is it always Wales?)

size of a cow (lol sorry couldnt help myself)

I would argue the bread box has faded as a size icon, but no suitable replacement has emerged, so objects of that size are quite awkward to describe in "as big as" shorthand.

First thing this makes me think of:
http://pregnant.thebump.com/pregnancy/pregnancy-tools/articles/how-big-is-baby.aspx

longer then the hat worn by dr.seuss

The period at the end of this sentence.

n/a that babby do be changing shape pretty radically each week!

A dinner plate.

re: football field, that seems to be the standard unit of measurement for amount of usable material harvested from a single baby foreskin when submerged in whatever mad scientist nutrient broth they use. a football field!

As big as a Buick.

i've got me a chrysler, it's as big as a whale!

and its about to set saaaail!

christ now i'll have that stuck in my head all day.

Kevin Blackwell once memorably described a footballer missing the goal by the margin of "a gnat's cock" in his post-match analysis.

A south African doctor described a tumour as being "about the size of a small football" to me (this didn't really make sense as a measurement in retrospect btw).

I think the length of a cricket pitch (22 yards) is used still.

Is there "the debt of a third-world country" still? haven't heard it for a while.

Sack of potatoes.

big as a baby's arm, hur hur

The area where they cut down trees was the size of five football pitches.

Belgiums are a common unit for measuring ice bergs and areas of rain forest.

I wrote a poem on a dog biscuit
And your dog refused to look at it
So I got drunk and looked at the Empire State Building
It was no bigger than a nickel

https://www.youtube.com/watch?v=wpmzS9mjz_I

Double-decker buses once a uk standard length &/or volume metric.

smaller than a baby's dreams

about as big as my love for robert fripp

"enough to fill (n) Olympic-sized swimming pools"

Missed swimming pools upthread.

Coins useful for hole diameters. When written, 'piece' used uncommonly often. 'the size of a 5p piece'. I see this a lot: "roll out the pastry to the thickness of a pound coin"

three fingers of gin.

http://freakytrigger.co.uk/ft/2007/02/the-all-new-si-units-of-measurement/

Manhattan is used when Belgium would lead to fractional units.

Distances in terms of airport runways.

"Did you know that 34 million American adults are obese? Taken together, that excess blubber could fill the Grand Canyon two-fifths of the way up. That may not sound impressive, but keep in mind that it is a very big canyon."

Ah yes, the advance upon X number of Olympic-sized swimming pools is filling a football stadium, often the Rose Bowl - in the USA.