If I wear Large size tee shirts, they are a little baggy, if I wear Medium, they are kinda tight, but not very tight, do the BDU jackets tend to be a little larger than a regular tee shirt? Should I get a Large or a Medium?
I wear a large, and I wouldn't fit in a medium Tee. When I was wearing Lg and not Lg-XL shirts, I had a medium blouse.
Why not try on a couple from squadron-mates and see which fits best?
Sizing charts are also available online.
I wear an extra-medium t-shirt (somewhere inbetween L and M) and my large blouse is juuuust a tad small in the sleeves.
Military clothing is notorious for inaccurate sizing. Different contractors use different stays/lasts when putting together clothing. They just have to stay within a specification. Same thing in civilian world. That's why Lee jeans fit differently than Levis for the same size on the tag. Try a couple of different brands of t-shirt. Always try on stuff(if you can) because even different runs from the same company can vary in size.