If by "sizing" you're talking about hide glue/rabbit skin glue, you can, if you're wanting to go traditional, use it for everything, as an alternative to epoxy. You need to use the right grade though, not just the canvas sizing variety that's sold for that purpose.
It is amazing stuff though, I think most here would be highly surprised by how strong a glue it is, and the advantages of it. Bear in mind, this is what's holding together 300 year old Stradivarius' under immense tension, that are still being carted around the world regularly and played by their current care-takers. The really nice thing about it is the ability to de-bond under controlled circumstances, which allows for easy repairs.
It's also the traditional adhesive for attaching gold leaf and tortoise, and that to whatever else.
The best epoxies on the other hand, often require the destruction of the material bonding it to remove it. Of course, if you don't have domed pins, or some other type of mechanical fastener, I'd say epoxy is mandatory, but if you are building something really traditional, you may want to consider the above.
Clear Silicone is a good option also, very few people use it in knives, and it's nothing like epoxy, but if you have something you want to adhere well, but only as a supplement to mechanical fasteners, like screws or domed pins, that can be taken apart for cleaning or repair easily, it works great, and cleans up super nicely.
It's a really great way to seal removable scales without making it a massive chore to detach later. It moves with the material, but has enough adhesion to keep from developing a gap between thin liners and whatever, as an example. I've used it in a few cases where I knew a customer wanted to use a higher end pocket knife for a while, and then have me clean it back up to presentation quality for their collection.
