For the longest time when tying large flies, I'd been focused on making them light. Relying on a large enough hook to keel the fly and, using water shedding materials and tying them in in such a way as to create profile or silhouette without real bulk. It works well enough that 30cm pike flies can be cast easily enough on a #10. I just can't bring myself to use a #10 for the seabass here and 20cm+ deep bodied flies are a bit of a push to say the least on the #8. I find myself topping out around 16-17cm, especially with scad or herring imitations.
So it was fish smaller flies or find a workaround. A couple of years ago I was mooching around looking at striper flies for inspiration and I stumbled across Mark Sedotti's work that really saved me a lot of fannying around and really shortened my learning curve. Put simply, it's the addition of weight to large bulky flies in order to counteract their wind resistance. At first reading I was a bit sceptical and, if I'm honest, a bit ideologically opposed to the idea of adding what amounts to casting weight to the fly rather than relying on the mass of the line. But I went down the river and had a mess around with some big flies and lead. Now I'm a convert.
Now, I should say that I'm talking about a lot of weight here, the hook in today's picture is a 4/0 SL12 wrapped with 0.030 lead wire and then keeled with 3x 4 lengths of the same wire. But fear not, the fly won't sink like a brick. The air resistance of the materials also creates drag in the water, so while there will be some action from the weight, you can counteract this with the materials you use in tying the fly. My first test flies didn't have the keel and swam OK. In fact they'd be ideal for still water fishing for pike, but the keeled versions swim better and will track true even in fast current- no fear of the fly's bulk turning it on its side and planing it to the surface. Not only that, I'm able to cast 25cm bulkheads quite comfortably with my #8, and they like casting trout flies on the #10... wel not quite but you know what I mean.
I'd certainly encourage you to have a play around with your bigger flies. It's quite easy to work out the balance that you like for a given pattern. Grab one that you've already tied, a roll of lead wire and head to where you practise casting (bring some water if you cast at a park) Add lead to the hook, say 10cm, wet the fly and cast it. Then add another 10cm, cast again and repeat. When you
feel you've gone too far try removing half of the last 10cm of lead and dial it in. When you're ready, keep it together so you can weigh it when you get home. It's not an exact science but you'll start to build a picture of how much weight you need to add to each fly. Adding anything like glass rattles, brass or tungsten beads and cones will all add up to give you your total weight and through experimenting with the type of weight and its placement you'll find a solution that you like for each pattern.
Here's a video of Mark tying a nice big slammer that looks to be close to 30cm long with a lot of weight on it too.