Blocks are generally denoted by length as a proxy for sheeve size. Sheeve size is determined by the diameter of the line, the number of sheeves is determined by the weight needed to be lifted. The number of sheeves will set the width; I've never seen a block listed by width dimension, but I suppose all things are possible.
To learn what size and style block is appropriate, I consult rigging books & photos of real ships, or look at photos of museum models. An excellent book is John Harland's "Seamanship in the age of sail." Another is "The Ashley book of knots." There are countless others, one's library can expand indefinitely
Be aware that museum models may not be rigged appropriately for the era, though. If the model is old, then it will likely have been re-rigged at some point to replace rotted line. The replacement rigging may or may not be correct. Once you find a photo or painting of the ship you are interested in, then compare the block size to a common object, eg. a man's head.
For working ships (merchant, fishing) the blocks will likely be less standardized in size; the ship owner is always looking to save money, and will buy what's cheapest at the time. There may be rigging losses during a cruise, and the replacement blocks will be what's on hand. For yachts, the cost is not so important, so blocks will be more likely to be "just right."
Hope this helps.