Creating custom size envelope SVG using Inkscape
Create rectangle 1.25" larger than your card
Create 4 squares 0.5" and place on 4 corners of rectangle and slice them out
Create a square, rotate 45 degrees resize to 0.25 larger than the card and slice the square in half to create 2 triangles (slice horizontal for top and bottom flaps, and slice vertically for side flaps)
weld the triangles to the main rectangle to create the card base
Create a new rectangle that is 0.25" larger than the card and center in the envelope. Rotate both by 45 degrees then group.
Save as plain SVG (add text with correct size if needed to let you know what size it is supposed to be when you import into other programs)