I want to mail a letter Registered, I have sufficient current Forever stamps, but apparently USPS says there's no way I can use them to pay the postage
page 41 for the allowed dimensions of a "letter" as opposed to a "large envelope (flats)"
You can then use a larger than #6 or #10 envelope (if your mailing is 3.5 oz or less). That will give you much more space for your stamps.
For Registered, no value, the fee is $14.65 and if having value the lowest fee is $15.25. To put that in terms of "Forever stamps" the first is 24 whole stamps plus 25 cents and the postage cost. The later is 25 whole stamps plus 25 cents and the postage cost.
Fitting 24 or 25 coil size (not commemorative) should be a breeze with planning. Write the addressee's address small but readable in upper case block letters. Do the same with you return address which can be placed on a back. You will be able to fit 4 horizontal rows of stamp, 5 or more if the envelope is taller than a #10. Six stamps in each horizontal row is no problem even on a #10 envelope. 4 rows x 6 per row is 24 forever stamps. Done carefully, you should leave enough room for the needed registry label. I have used a #10 for registered mail with forever stamps, but a larger envelope is easier.
If this is not going to be a one time mailing, but you will be doing regular registered mailings and you are concerned about space pick up a few dollar values, $1, $2 or $5 of the regular issue size at a discount or even at face. That will cut down on the number of forever stamps you need to use (taking less space).
Also you need not use just your stamps if there is a lack of space, put on what you can leaving space for a meter strip which pay the rest due.
Finally if your mailing are over 3.5 ounces or are in the large envelope size range using forever stamps becomes easier as you have more space which will allow for some commemorative size forever stamps to be used. Remember the the commemorative stamps take twice the space as the coil forever stamps.
Lastly I was once asked by a window clerk if I actually expected her to count up the stamps on a large envelope. Yes, that is why each winder clerk has a calculator function their computer screen, or you can take may word for it. She used the calculator function. Any pre-1975 clerk would have been faster with their adding machine and the real old timers would just count them up in their head or with a quick note or two on the envelope as that was their calculator of the day.
Here is a registered I sent today: