Is size a word in the Scrabble Dictionary

Yes, size is playable

...and is worth 13 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Size" Definition

noun

Any of a series of graduated categories of dimension whereby manufactured articles, such as shoes and clothing, are classified.
Any of several gelatinous or glutinous substances usually made from glue, wax, or clay and used as a glaze or filler for porous materials such as paper, cloth, or wall surfaces.
Character, value, or status with reference to relative importance or the capacity to meet given requirements: Try this proposal on for size.
Considerable extent, amount, or dimensions: a debt of enormous size.
Relative amount or number, as of population or contents: What size is Cleveland?.

phrasal-verb

Size up To make an estimate, opinion, or judgment of: She sized up her opponent.

adjective

Sized. Often used in combination: bite-size appetizers; an economy-size package.

verb-transitive

To arrange, classify, or distribute according to size.
To make, cut, or shape to a required size.
To treat or coat with size or a similar substance.

verb

To make a particular size bring to proper or suitable size.
To arrange, grade, or classify according to size or bulk.
To equal in size or other particular characteristic .
To cover, stiffen, or glaze with or as if with size.