All right, so here's that size index I promised, in approximate measurements of body length (not including legs) and weight. For those suffering under the cruel oppression of the metric system, I have included conversions that none of you will find helpful but will further my lifelong goal of ridiculing your measurement system.
Massive Heretic
30 inches long, 16 lbs (Metric: 5 kilo-somethings)
Ryu
25 inches long, 14 lbs (Metric: 20 parsecs)
Guile
22 inches long, 14 lbs (Metric: 1.9 Parthenons)
"Mid-size" spider
10-14 inches long, 4-7 lbs (Metric: 0.3 shillings)
"Small" spider
Up to 5 inches long, 1 lb or less (Metric: about the size of Nicolas Sarkozy's nose)
And then, we have Keito
5 feet, 7 inches tall, maybe 85 lbs? (Metric: think Poland)
And finally, Boss Spider
16 feet long, ~250 lbs (Metric: 6 autobahns)
Of course, the real problem is putting those measurements into the story in some way that doesn't feel completely out of place (it would be silly to have Guile jamming out precise measurements, or even specific species of dogs for comparison). Hmm...