In the Monster Manual (p. 308) it states:
The yuan-ti can use its action to polymorph into a Large snake...
Can the snake be as large as I want or is there a set size? The description wasn't specific.
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In this context "Large" means of the Large size category (Player's Handbook p.191), meaning it occupies a 10' by 10' space. The exact phrase is the following:
The yuan-ti can use its action to polymorph into a Large snake, or back into its true form.
Note that the capital-L Large refers to a size category, not a general indication of bigness.