The factory door speakers are 6.5". You can use a 5.25" speaker, but you'll need to make an adapter plate. Pretty simple to do.

 

 

Crutchfield has the adaptor plates if you'd rather get them pre-made. Just select the year and your blazer , 2-door or 4-door etc..

You'll find the plates.

El-Beautor is just trying to help you save some change because they are around $20 a set..

 

These are for a 97 4-Door Blazer:

https://www.crutchfield.com/g_103200...s.html?tp=2824

If your configuration is different , then just make the required changes on that site..

 

If you want to make your own you can do it from ABS plastic, or MDF board. If you use MDF or any other wood product you should also coat it with something to water-proof it as it will be exposed to at least a little humidity/moisture in the door. Just use the factory speaker as a template to cut the outside ring, then use the template with your aftermarket speaker to make the inside hole in your adapter ring.