Webbing versus Rope
First, webbing isn't as strong as rope generally. While some companies, notably Ankarolina, tout their webbing as being as strong as 1" diameter rope, I'd point out that the breaking strength of their webbing is about equal to the WORKING strength of the 1" rope and has no safety margin factored in.
Also, webbing is far more susceptible to chafe.
Before considering what size docklines to use, it might be wise to consider the cleats that the docklines are going to go to. The rule of thumb for cleats is that they should be SIXTEEN times the diameter of the lines normally used on them. This will generally allow you to use TWO docklines or ropes on each cleat without problem.
If you plan on using 5/8" line for the docklines, you probably should have 10" cleats... and if you use 3/4" docklines, 12" cleats are a good idea.
Type of Rope
Using old halyards for docklines is generally a really stupid idea IMHO. Halyards require different stretch characteristics than do dock or mooring lines, and are generally made of the wrong materials to be good docklines.
Most halyards are polyester double braids, and after being used are going to be weaker than nylon lines of the same diameter. They will also be far less elastic, and provide less shock absorption, putting a greater shock load on the deck hardware in a storm.
Again, IMHO, it is pretty stupid and penny-wise, pound-foolish to skimp on docklines. Like ground tackle, the docklines are often what you're depending on to keep your boat safe in bad weather. A good nylon rope is what you want for docklines generally.
Docklines can be one of three flavors....
are the strongest, but have the most trouble with rough pilings and such, since the splinters and such tend to pick the fine strands apart. They also have the least stretch. However, they don't hockle or twist as easily as three strand, so may be the best overall dockline for general use.
lines do better with rough pilings and such. However, three-strand lines are far more prone to hockling and kinking. They do have better stretch characteristics than double braids.
Octo-plait or twelve-plait
has the best elasticity of the three types of docklines, but are fairly rare and a bit more expensive from what I've seen. It does stow more compactly than three-strand line, and has some of the better characteristics of both three-strand and double braid. Like a double braid, it doesn't tend to hockle or kink. Like a three-strand, it handles rough pilings fairly well and stretches fairly well.
Woven chafe protection sleeves, like tubular webbing, are by far the best form of chafe protection IMHO. While not as durable as vinyl, plastic or rubber hose, it allows water through to cool and lubricate the docklines. One of the most common forms of line failure is due to internal friction and heat melting the line and causing it to fail. This is generally exacerbated by hose type chafe protection, since they trap the heat and prevent the water from getting to the line. Spectra or dyneema chafe sleeves are probably almost as durable, if not more so, than the hose chafe protection, yet allow the heat to escape and the water in.
Sizing the line
Going slightly oversized on docklines is a good idea. Don't make the docklines too thick, since they will be harder to handle and not provide the elasticity and shock absorption that is really ideal in a good dockline.
Yours is a big boat, and 1" lines aren't unreasonable. A 1" three-strand nylon line has a breaking strength of somewhere around 29000 lbs. Your boat weighs 30000 lbs... and if you factor in shock-loading during a storm, 1" lines are pretty close to the minimum I'd recommend for it.
That also means you need at least 16" cleats.
If the docklines are properly sized and protected from chafe, snubbers really aren't necessary IMHO.