With all sweater patterns, the amount of "ease" or looseness is a personal preference. Negative ease is a measurement smaller than your body measurement to create a tight fitting garment. Positive ease is a measurement larger than your body for a looser fit. The best way to decide on any size is to measure a sweater that you own (whose fit you like) and check the pattern schematic or measurements to choose a size you like.
Here's a general fit guideline:
Very close = 0 ease
Close Fit = chest + 1 - 2 inches ease
Standard Fit = chest + 2 - 4 inches ease
Lose Fit = chest + 4 - 6 inches ease
Oversized = chest + 6 inches and up
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