Those fairy lights have their place, but there are a number of issues that make them less than ideal for what you’re wanting to do.
If you wanted to cover a 100cm square at 5cm spacing, you could weave them back and forth in a zigzag but you would need about 400 pixels, which is a problem because:
- The addressing doesn’t work the same as WS2812B-compatible LEDs; each LED has a fixed address and each string starts over at 1 (or sometimes a higher number, if they chopped a few off the front of a reel). If you connect multiple strings together, the portion of the pattern on the first string will be echoed on each successive string.
- There’s a severe voltage drop from one end of the string to another which will cause color shifts and can cause occasional malfunctions (eg. a bright section of a pattern could cause a “brownout” and make the string stop listening). Some people on the forum have suggested that the effective limit is about 200 pixels.
Also, the fairy light LEDs are coated in some kind of translucent resin (possibly hot glue) which diffuses the light well but also reduces the brightness, which may be a problem if you want the matrix to be visible from a distance.
To make a large matrix, @jeff used a mesh net of 12mm bullet-style WS2811/WS2812s. I’ve also seen people drill a grid of holes in a board and populate it with the same 12mm bullet-style LEDs, or cut a reel of 30 or 60 pixels-per-meter LEDs into strips and mount them in parallel lines on a backing board.
To make the LEDs visible from a distance, I’d suggest going with a backing-board design and then using some kind of diffusing material to make the apparent size of the pixel larger. Some people glue ping-pong ball halves over each LED; others put a walled partition (like the dividers in a carton of wine bottles) over the LEDs with a translucent material across the top.