Find out the max drag of your reel in the size needed. Let’s say your reel does 10kg drag. You then know, that if you do not thumb the reel adding extra drag you only need 10kg line as this is the max pressure it will need to take. For safety, I go slightly heavier as you often over tighten the drag on big fish or thumb the reel. By doing this, you will fit the most line you can on your reel and may find you don't need as large a reel as first thought. REMEMBER you can use a much heavier trace line if needed, but the main line only needs to stand up to the reels drag pressure and no more. As such, there is really no need to go much heavier unless you intend to skull drag the fish from the bottom. If you risk your line getting cut on sharp reef or alike then top shot 10m-15m of a heavier line.
Once you know the lb of the line your using I recommend to people for the safety and best usability of most rods, get one that is 25% stronger/heavier than your line. With a stronger rod taking more weight than the lighter line you can now lift your rod angle higher than if lb of lb in-turn getting more line back in each lift. BUT won't I break my line now? Not if your drag is set right.
Whilst having a heavy line is nice for peace of mind, it is not always the right choice and with sports fishing growing fast, generally people are going lighter and lighter in line class, hence lighter in rod weight. In some instances, say fishing on a reef/structure you need to fish heavy as that is your only option to stop getting busted off. This is about the only advantage of fishing really heavy gear. Generally speaking lighter line is thinner and as a result it will cast further due to less line friction on the guides and drag through the air. This is why a braided line will cast further than mono of the same LB as its much thinner.
So what do I need you ask? Well, two things will mostly decide this. First the size/type of fish you’re chasing and second where you’re chasing them. As mentioned, when fishing structure it is often not possible to truly sports fish and a minimum of 30lb-50lb is recommended depending on the fish/size. But then for bream fishing and alike 6lb gear or the lightest your game to go really is recommended.
I like sports fishing as such and the following is what I use.
My all-rounder rods are around 6kg/12lb sticks loaded with a 10lb braided main line. I have used them for trevally to salmon to jewfish and have also pulled kings up to 108cm on this light tackle. It is fun and about all that most will need when starting. When fishing for larger fish without the risk of snags and structure I use these rods also. REMEMBER, you can always use a heavier or lighter trace line to suit what you’re fishing for.
When fishing on structure for kings or similar fish that pull, I run 30lb-50lb gear which can still be to light at times.
When game fishing, I run 15lb-30lb gear stand up but up to 80lb - 100lb when needed.
This really nowadays FALLS ON YOU as the fisherman to find what direction you will head in.
The Sydney Marine Man