Amazing Spider-Man #18, page 15, panels 1-2
Amazing Spider-Man #18: Flamin' 'Eck 56
Written by Stan Lee, Author of The Fantastic Four
Illustrated by Steve Ditko, Illustrator of Dr. Strange
Lettered by Sam Rosen, Letterer of… Patsy Walker?!!
I can give a pass to Johnny using his flame to create a giant '4' logo above the centre of New York to summon the nearby members of the team. I'm a lot less likely to do the same for an entire message, written in the sky above the centre of New York, that can be read all the way out in Forest Hills.
Traditional skywriting letters are approximately 3000 feet tall, and last for no more than a couple of minutes. These letters are approximately two feet tall, and no indication is given as to how long they last. Assuming that they behave similarly to their smokey counterparts, then it's very lucky that Peter wasn't somewhere out of sight of the sky for those couple of minutes. Like, say the little Spider-Boy's room. And let's not get into how great Parker's eyesight is, being able to clearly read these letters from a distance of about 10 miles...
Check out our coverage of Amazing Spider-Man #18 on our thirty-sixth episode: John Byrne Quits Comics