There are a lot of options! Velcro itself comes in a bunch of sizes and shapes, but most pedalboards use the 2" wide strips of industrial strength velcro (It doesn't matter what color because the Velcro sits under your pedals!).
This type of Velcro works very well because it is nice and wide and fits most pedals nicely, especially Boss pedals.
Velcro is pretty expensive, costing about $30 to cover an entire pedalboard, and you can't really re-use it.
If you decide to add or remove pedals to your setup, you have to buy more Velcro.
Another problem with using Velcro is that you have to rip the bottom material off of your pedals so that the velcro can attach directly to the metal of the pedal.
This damages your pedals and significantly lowers their resale value.
Think about it, would you want to buy a pedal that has goo all over the bottom of it? Probably only if you get a good low price.
Also, Velcro wears out and eventually the pedals can fall off of the board entirely.
This thwarts the entire idea of a pedalboard! There is a company called Godlyke that has made a Velcro alternative called Power-Grip.
Under a microscope, you can see that power-grip uses interlocking mushroom teeth instead of the hook and loop method of Velcro.
Power-Grip still adheres to your pedals with a sticky goo, and you still have to pull the rubber off of the bottom of your pedals to install it.
If you can believe it, it is more expensive than Velcro.
One popular alternative to velcro is attaching links of a bike chain to the pedals and using the bike chain links to screw the pedals down.
This is a great method that works very well.
You have to buy a cheap bike chain (or take one off an unused bike) and buy a bike chain tool (about $10).
Use the tool to dismantle the bike chain, and then you can attach the small figure 8 parts of the bike chain to the existing screws on the bottom of your pedals.
Then you can flip the pedals over and screw them down.
It does not damage your pedals and does not hurt their resale value.
Unfortunately, this method will not work with all pedals, particularly custom and boutique effects.
There is one small company called Stompsters that are selling just the parts of the bike chain that you need, along with screws.
They are cheap and effective.
Of these three methods, I prefer the bike chain method.
It is cheap, easy to install, and it doesn't damage my pedals.
For pedals that won't work with the bike chain method, I use Industrial Strength Velcro because I can get it at any hardware store.
That's about it for pedalboard velcro - happy DIY Pedalboard building!