I haven't done any surfcasting in years, but a 3oz sinker wouldn't get me to 100m. Hopefully someone more experienced than me can help

I'm not much of a surfcaster, but if distance is important, line and rigs make a big difference. You will cast further with braid, but it has it's downsides. It has poor abrasion resistance and because it doesn't stretch, it's harder to keep sinkers stationary in a tidal flow. Regardless of main line type, you should use a shock leader, 10lb breaking strain for every 1oz weight is a rough guide. Small baits and clipped down rigs are the way to go for distance. Big reels with long spools will also help.

A 14ft will be fine once you've cast a few times.

Pcj wrote: Go down to a rugby field and practice casting. Amazing how far a sinker can travel when line breaks.

I use to use about 1 and 1/2 rod lenghts

I doubt that braid is too light but I'd make the shock leader about 2 rod lengths or at least long enough so there are a few wraps on the spool when ready to cast. Post up a pic of your FG knot, I think it will be a little too bulky. What are you using for a shock leader Jo Bloggs? I would think 24kk mono would be good.

Back surf casting,10lb mainline and 20lb absorber.   nylon,no braid back then

20lb main line to 80lb shock leader is a huge leap especially if you are considering any distance and that doesn’t matter how tidy your knots are.
Have you considered tapered leaders?
A couple of wraps on your reel then whatever you are comfortable with outside your rod to cast is all you need.
My lighter 14’ rod I’m running 12lb braid to 30lb tapered leader

Are you going to try under harbour bridge in weekend. If so can easily take a drive there, and refine those knots. And can maybe show you how to get a bait to go a bit further out.

By the way, you can cast 3 ounce a long way if line is light and bait is very streamlined.