The recommended map sizes typically fall into 3 categories: Rushing, Balanced, and Extended. Think of this as short, medium, and long in regards to the amount of time before the first attack and also in regards to the length of a game.

Available Map Sizes (for rated games)
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320 x 320 Map Size
384 x 384 Map Size
448 x 448 Map Size
512 x 512 Map Size
584 x 584 Map Size
640 x 540 Map Size

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     | 320x320 | 384x384  | 448x448  | 512x512  | 584x584  | 640x640   |
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1v1  | Short   | Medium   | Long     | Long     | Long     | Long      |     
2v2  | Short   | Short    | Medium   | Long     | Long     | Long      |
3v3  | NA      | Short    | Short    | Medium   | Long     | Long      |
4v4  | NA      | NA       | Short    | Medium   | Long     | Long      |
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  * NA = Not Available

Increasing the map size basically increases the length of the game. Newer players should use larger map sizes in order to give themselves time to build, and to properly prepare defenses against their opponents.

Any player or team that wants to rush should always use the smallest map possible. A smaller map allows you to attack your opponent more quickly.