Found some box and envelope sizes on Ebay:
You can buy here at this link: (cut and paste into browser)
https://www.usps.com/ship/prep... (select URL, open right click or copy paste)
Here are the package details and sizes available:
1 Priority Mail Small Flat Rate Box (currently $5.35 at retail; or less if you pay for postage online) – PPSFRB - Size: 8 5/8" x 5 3/8" x 1 5/8"
2 Priority Mail Medium Flat Rate Box (currently $11.35 at retail; or less if you pay for postage online) – PPFRB1 “cube style” - Size: 11" x 8 1/2" x 5 1/2"
3 Priority Mail Medium Flat Rate Box (currently $11.35 at retail; or less if you pay for postage online) – PPFRB2 “shirt style” - Size: 13 5/8" x 11 7/8" x 3 3/8"
4 Priority Mail Large Flat Rate Box (currently $15.45 at retail; or less if you pay for postage online) – PPLFRB - Size: 12" x 12" x 5 1/2"
Prepaid Forever Priority Mail Flat Rate Envelopes
5 Priority Mail Flat Rate Envelope (currently $5.15) – EP14F - Size: 12 1/2" x 9 1/2"
6 Priority Mail Legal Flat Rate Envelope (currently $5.15) – EP14L - Size: 15" x 9 1/2"
7 Priority Mail Padded Flat Rate Envelope (currently $5.15) – EP14PE - Size: 12 1/2" x 9 1/2"