I trimmed the cards lengthwise so that they were about ¾” tall by 3.5” long. This made the spacer high enough that I could grasp the spacer firmly, but not so high that it made the work awkward, and wide enough that I could glue several bricks in place before I needed to move the card over to lay more bricks. I also made a second spacer about ¾” by about one inch. I inserted that between the bricks so that I had consistent vertical and horizontal spacing throughout the project. I made two of those shorter spacers.
Each time I glued a new brick into place, I pushed it up solidly against both the horizontal and vertical spacers. The reason why I made a second, small spacer was that I needed to have the glue become fairly tacky before I pulled out my first spacer. By the time I had glued in a second brick, the glue on the first brick had usually become tacky enough that I could pull out the first spacer and move on to lay a third brick. And on it went.
I found only two disadvantages in using these paper spacers. One problem is that the glue occasionally seeped out from the edge of a brick and adhered to my spacer. (I had to replace one of my spacers after doing about half of the bricks.) The other problem is that the spacer somewhat obstructs previous rows of bricks laid. Once during the process of doing my wall, I had laid about four or five bricks before I noticed that I had not staggered the first brick of the row using a half brick. (My spacer had obstructed my view of the row just below it.) I ended up having to tear out the row and start over. Aargh!
I hope this answers your question, Lisa. Thanks for asking!