For those of us who don't use the metric system:
1) the 15cm x 15cm card is approximately a 6"x 6" card
2) the 21cm x 21cm square to start the box, is slightly... and I do mean
very slightly... larger than 8.25" x 8.25", so an 8-1/4" square would
3) and when she scores the 2.5cm edges, that's about 1" (1" = 2.52cm)
Hope this helps a little with the conversion to inches!
Watch on YouTube: HERE
She makes lovely boxes to match her cards all the time... I should do this, too! :)