We received a quote request recently that included an outer envelope measuring 4″ in height by 9″ in width. We suggested to the company that they adjust these numbers a bit to a standard #10 which is 4.125″ in height by 9.5″ in width.
If possible, always go with standard size envelopes because it will save you money.
Reason being, many printers place bulk orders for standard size envelopes and store them for when needed. Placing the orders in bulk means they get them at a lower price which results in YOU getting a lower price.
Custom size envelopes usually have to be created for that particular project…and this obviously comes with a higher price tag.