What size window envelope should we buy when using voucher checks?

For a review of Quicken - compatible check styles and perhaps to order new check or envelope supplies, please see https://intuitmarket.intuit.com/checks and https://intuitmarket.intuit.com/envelopes/check-window (the website talks about checks for use with QuickBooks but the same style checks also work with Quicken) or order from any check printing company that offers Quicken/Quickbooks - compatible checks and envelopes.

According to the envelopes webpage above, Standard Voucher Check envelopes should do nicely, as well as the Self Seal - type voucher check envelopes.