The 8x8 would make a nice kit but its lack of built in cpu means it would be a slave to some other board.
Hand wiring scads of LEDs by is not fun nor durable. And by not using too many LEDs makes the kit affordable and simplifies the both the driving circuits and power supply.
The 7x24 would allow for four 5x7 letters (with 1 space between) and looks like a nice size, I would have liked 8x24 but text seems to be the use of choice.
Since the design uses normal 5mm LEDs it will only be seemlessly expandable side by side. 24LEDs running at up to 60ma each can draw nearly 1.5 amps so a switch mode regulator may be called for, but a linear is cheaper and more common an will run hot. Reducing the current per LED will help, 30ma @ 1:7 duty will be bright even with standard LEDs and draw a less costly 770ma