Size which had been worrying me for some time. It is a rife word and yet I hadn’t found any word I liked to overset it. I had been saying greath for a time—that is great with –th—but it sounded clumsy. And for some unknown I had not seen the most obvious word for it…until today. I was reading a book with a quote from the 1700s where somebody was describing a thing and spoke of its bigness. The word seemed so natural and flowed well with the other words, and of course it was easy to know what it meant straight away. Even though you might think it would sound odd to speak of the bigness of small things, we already say height even of low things and length of short things. Size is a small gap filled with bigness.
Roots English 
I’ve got ‘bigness’ as a swap for ‘size’, but I think it only sounds right sometimes. It wouldn’t work well for something like: “what size shoes do you take?” for instance. There’s another word lurking about somewhere that we haven’t seen yet! 🙂
Wordmeal, my partner in her mangled pidgin English says “What number shoes…?”. Of course, number is not pure English either. Personally I have no real issue with “size” (or “number”, for that matter!)
Likewise, I’ve been saying “hotness” for “temperature” since I was a small child, even. Why is it that the form for the most is the default one, here? (hotness, height, length, bigness, and so on?)
Also like to mention the use of “heigth” /haiT/ which still lingers; “greath” I would imagine as being said without the t of “great”, that is, /greiT/
I don’t know why the “most” of two adjectives is chosen, and I cannot even come up with a guess. I wonder what other tongues do? Maybe we’ve stumbled upon some deep linguistic law!