There are 9 typical sizes for formal/informal invitations, greeting cards and announcements. There are also specific envelope sizes that correspond with these sizes, they are called "A Series" envelopes. Below are the most common sizes:

4 Bar Envelope
MEASURES: 3.625" x 5.125"
ENCLOSURE: 3.375" x 4.875"

5 Bar Envelope
MEASURES: 4.125" x 5.5"
ENCLOSURE: 3.875" x 5.25"

A-2 Envelope
MEASURES: 4.375" x 5.75"
ENCLOSURE: 4.25" x 5.5"

A-6 Envelope
MEASURES: 6.5" x 4.75"
ENCLOSURE: 6.25" x 4.5"

A-7 Envelope
MEASURES: 7.25" x 5.25"
ENCLOSURE: 7" x 5"

A-8 Envelope
MEASURES: 8.125" x 5.5"
ENCLOSURE: 7.875" x 5.25"

A-Long Envelope
(Slimline)
MEASURES: 8.875" x 3.875"
ENCLOSURE: 8.625" x 3.625"

A-9 Envelope
MEASURES: 8.75" x 5.75"
ENCLOSURE:8.5" x 5.5"

A-10 Envelope
MEASURES: 9.5" x 6"
ENCLOSURE: 9.25" x 5.75"