The tool rests that come standard with most lathes are generally between 6 and 12 inches long. These tool rests are probably fine for turning small things, but the first time you try to turn a chair leg, or anything else longer than the tool rest, you will immediately recognize its limitations. These short tool rests force you to reposition the rest and turn your workpiece in 6" to 12" segments. This prevents you from making the full body fluid motions that yield consistent, smooth, flowing turnings. It is possible to find commercial iron double post toolrests that are 24" and longer. They are nice, but can be hard to find and are usually pricey. I use three simple wooden tool rests for all of my turning. I put the tiny rest that came with my lathe in a box about a year ago, then I forgot where the box was. Good riddance.
Curtis Buchanan taught me to turn during a fall and winter that I had the good fortune of working with him in his Windsor chair shop in Tennessee. My tool rests are based on the ones that Curtis uses. However, lacking the 3" thick maple butcher block that Curtis used as the base of his rest, I had to come up with a slightly different construction method for mine.
I wanted to design a tool rest that was easy to build from common materials. My tool rests are made of three components. The base, which rests against the bed of the lathe, is a piece of spruce or fir construction lumber roughly 1.5" thick and 6" wide. The portion of the rest that the tool rides on is maple (any diffuse porous hardwood is fine). Finally, a beveled glue block adds rigidity to the structure. The construction is simple and requires no fasteners. The components are simply planed and then glued and clamped together. The large glue surface area creates a rigid, vibration free tool rest.
Two slots in the base accept the bolts that clamp the rest to the bed of the lathe. The slots allow adjustment toward and away from the lathe centers to accommodate workpieces of different diameters. The bolts are threaded into wooden blocks with T-nuts embedded in them. The blocks slide under the ways of the lathe and clamp the toolrest to the lathe bed when the bolts are tightened. On my 24" toolrest the slots in the base are 3" from the ends of the base piece. On my 45" toolrest the slots are 12" in from the ends. The softwood base does dent under the washers from the clamping pressure of the bolts. This has no effect on the performance of the tool rest, but if you wish to prevent or minimize this denting, then a harder wood for the base is the ticket.
The dimensions of your tool rests will be determined by your needs and your lathe. In the photo above you can see that the dimensions of the end of my tool rest were determined by my lathe's tailstock. I wanted the tail of the tool rest to be as long as the lathe would allow so that the tool rest can accommodate as many different workpiece lengths as possible. The limiting factor for the length of the tail of my tool rest is the tailstock crank.
The tail of the tool rest is the portion most vulnerable to vibration. Because of the cutout the tail of the tool rest is not supported by the base. The cutout provides clearance for the base of the tailstock and access to the bolt for tightening and loosening the tailstock to adjust its position. The portion of the tool rest above the cutout should be as wide as possible to minimize vibration in use. The tool rest pictured above has a full 2 3/4" above the cutout.
The total length of your tool rest is whatever your heart desires. I use a 16 inch, a 24 inch, and a monster 45 inch tool rest. With the 45" long tool rest I can turn the rear legs of ladderback chairs without repositioning the rest. Each tool rest cost less than $10 in materials and about one hour to build.
The height of the top edge of the tool rest should be somewhere below the height of your lathe centers. How far below depends on personal preference. All of mine are slightly different and I have no preference between them. The top edge is beveled back at about 65 degrees. This keeps the contact point of your tool close to the wood. When I bevel the top edge I leave a 1/4" flat at the apex and then slightly round it to create a durable contact point. If a tool (read skew) catches and gets slammed into the tool rest, it will dent. When that happens I plane the dent away, then recreate the bevel and rounded top edge. It is important that the top edge be straight and smooth. When I round down piles of green wood the wet shavings and sap tend to accumulate and gum up the top edge. If I have more green wood to round down I use a cabinet scraper to quickly scrape the gunk off of the top edge with the tool rest in place on the lathe and then get back to work. When I finish turning, I plane and reshape the top, a five minute affair.
The slots in the base piece are the most time consuming part of building these tool rests, aside from waiting for glue to dry. I drill out a row of overlapping holes on the drill press and chisel out the remaining waste. I also make a cutout in the beveled glue block to allow the rest to be adjusted closer to the centers than would otherwise be possible.
I have been using these tool rests almost every day for the past two years. They are as functional as the day that I built them and I couldn't imagine working without them.