"Alles" means "everything" in this instance. One only uses the other forms of "alle-" when it is modifying the noun, that is to say, being used in the sense of "all of the <noun>". In that instance it is declined as shown here:

http://en.wiktionary.org/wiki/alles#German

"Die Grösse" is a noun meaning "the size " http://www.canoo.net/inflection/größe:N:F

The adjective "gross" meaning "big " changes forms depending on the noun's gender, number and case and for whether there is a preceding definite article (der, die, das, die), indefinite article or possessive pronoun (ein, eine, ein, eine, kein, mein, etc.), or no articles

http://www.learn-german-smarter.com/learn-German-adjective-endings.html

The comparative "bigger " or "grösser" also changes like any adjective to match its noun. The superlative "biggest " or "grösste" again changes for gender, number and case and article

http://www.canoo.net/inflection/groß:A

Größe=size and Maße=measurements. One is singular and the other is plural, and they have slight differences in both languages.