Even though there are standard dimensions for some typesof packages, it's advisable to check them against the diemnsions in the actual datasheet of the chip. For example, my favorite common pitfall is the width of the SOIC chips.
you can refer ipc standards for the footprints,but then goin thru the data sheet for the particular component is recommended as told by kender,visit www. pcblibraries.com website and ask the query to Mr Tom Hausher,he might help you out on this regard.Hope this may help you too.