So i have been in an accident and broke my left arm. And i started working out and noticed my left chest is smaller. Would my chest be even if i just keep on working out or do i need a specific routine ? Thank you
No, you do not need a specific routine to even out a muscle imbalance, and attempting to do so will likely only make it worse.
Here's why you don't want to do specific work to correct the perceived strength imbalance: Let's say you have a left pectoral that is weaker than the right. So you do a bunch of dumbbell chest flies and bench presses with the left arm only. Now the biceps, triceps and deltoids are stronger on the left arm than the right, because you inadvertently strengthened them as well when trying to correct the pec muscle imbalance. And what if you got it wrong in the first place, and it wasn't actually a muscle imbalance causing the observed size difference? There's just way too much to go wrong with this kind of approach, and it's too difficult to assess whether you're getting it right.
Instead, do exercises that require even use of both arms. The weaker arm will limit how much weight you can lift, so the stronger arm isn't going to get any stronger than that, and they will very quickly even out in strength.
For the chest, you would do this using a barbell bench press, or if that isn't an option, then a bench press machine that allows the left and right arms to move independently. In either case, you need to make sure that your hands are always at the same height - so you don't push harder first with your stronger arm. Do not use a Smith machine or a bench press machine that has the left and right grips fixed together, as these allow you to push more with one arm than the other. (Also be sure to read this previous post on bench press safety.)
I think you are rookie enough not to be too worried. Doing dumbbell press will typically let you know if you have any major strength imbalances. I would suggest you do more dumbbell work if you are really worried, but at this stage it is more important that you learn to do movements safely.