He is Armor Class 3, plate mail, has 1 hit die and a poisonous bite.The final text is significantly emended:
He is armor class 3 (plate mail), has 6 hit dice (31 hit points), and his bite causes 1-8 points of damage and is poisonous (-1 on saving throw dice because it is so strong)For a first level group, that is straight-up nasty: 31 hit points takes 9 average hits to kill, and at AC 3 in Holmes that takes 36 attacks. It's quite likely to be a TPK unless the PCs just run from the encounter. Certainly a good lesson, but it points to the giant spider becoming standardized as a type.
The spider pictured above is a Brazilian wandering spider, reputedly the most venomous spider in the world. The largest is the giant huntsman spider, whose leg span reaches up to twelve inches. A Goliath birdeater tarantula only goes up to 6 ounces, and that's the heaviest spider by weight. So that gives us parameters for spiders.
Humans are the archetypal 1 hit die creature. At minimum, an adult human is about 4'10" tall and weighs 100 lbs. According to the square-cube ratio, if we took our maximum spider and made it 3' in width, that would be over 100 lbs already, and at 6' it would weigh almost 500. Compared to a person, I think that's enough to merit extra hit dice!
But Holmes's giant spider could be the more modest 3' wide spider. That's still a mean critter, but it is a sensible 1 hit die. We could scale up by a hit die per foot of leg span, with a 6' spider being 4 HD, and the monstrosity in the Blue Book at 8' and over 800 lbs. This gives us a good scale for giant insects in general, and allows for a flourishing variety. Maybe the spiders on your 3rd dungeon level have 3 hit dice. That would make them 5' wide and weighing as much as an NFL offensive lineman (over 330 lbs.). Similarly, we can change the effects of their bite, or even make them non-poisonous (if such a thing can be imagined).
Here's a chart based on some quick-and-dirty math:
|1'||6 oz.||1 hp|
|2'||14 lbs.||1-2 hp|
From here, you can get an idea of the size and hit dice of any giant insect or arthropod you care to put in your game.
Spitder Image by Techuser, CC-BY-SA