There are several preferences when it comes to pin sizes. Some will prefer 23g all around, some will go with 1" all around. Some even prefer to use insulin pins all around. All of these will get the compound into the muscle, if used properly. Below are my recommendations for the beginner that is still trying to adapt and learn their body.
The gauge size determines the flow rate of the injection compound. The larger the hole, the faster the flow rate. The faster the flow rate, the quicker it enters the muscle and the greater chance of it to sit in one spot. Reducing the speed of the flow, either with the plunger or the pin size, will allow for the compound to spread out more evenly. For the beginner, this is important because you are injecting a foreign material into the body and the body is not used to seeing this. Therefore the likelihood of experiencing PIP is higher for you than a more experienced person. You don't have to time it, but a flow rate of approx. 30 sec per cc would significantly help reduce PIP.
The pin size matters depending on the muscle being injected. The glutes are the largest that you will inject, but they are also the deepest. Even in a fairly lean individual, a layer of fat must be penetrated prior to entering the muscle. Ideally, you should try to get towards the center of the muscle being injected. This is where the pin length comes into play. Smaller muscles and those with less fatty tissue can be injected with shorter pins.
Glutes: 25g, 1.5"
Quads: 25g, 1"
Delts: 25g, 1"
Note: I chose 25g for the first timer because it will help to control the flow rate to a point that will help to reduce the PIP for you. Otherwise, 23g could be used for quads and glutes.
The larger the muscle, the greater the amount of liquids it can hold. Likewise, the smaller the muscle, the lesser amount it is capable of sustaining.
Site Locations & Rotations
When you begin EOD pinning, you're going to want to rotate your injection sites in a manner that allows each pinned muscle time to rest and recover from the stress caused by introducing the foreign matter. This means to avoid pinning the same muscle twice in one week. With good rotation of just glutes, quads, and delts, on a basic cycle with a prop taper and/or kicker, you can accomplish this. This link is a good resource for you to learn site locations: