Pronounce it just the way it's spelled:=)
You already made it easy for yourself by separating it into smaller portions. :)
The p in front is silent, and "pneumono" would be pronounced just like pneumonia except the "ya" or "eeya" (depending on your accent), would be replaced with "o" as in "oh no!" (pneumo can mean things related to lungs, or a rod-shaped bacteria...given the length and content of the rest of the word, I'm going to guess it's a bacteria). Ultra Microscopic (pronounce them just like the individual words...microscopic meaning smaller than you can see with the naked eye, and ultra emphasizing even smaller than that...teeny tiny. Silico just like in "silicone sealer" (i.e., long "o" at the end), meaning contains silicon. Volcano would be pronounced closer to "vulcanized" (as in the rubber) than the thing that spews lava (not sure the meaning in this word...either something that comes from a volcano, or found in other extreme temperatures I guess). Coniosis would be broken down to the syllables: co- (not sure if it would be a long or short O...maybe long, like in co-worker), -ni- (like knee), -o- (definitely a long O on this one), -sis (just like sister). (not sure that the coni part means, but any -osis or -iosis usually implies a pathology/illness/disease of some sort).
So, you have a disease caused by a teeny, tiny, rod-shaped bacteria that either has silcon incorporated into its cells or lives in an environment with a concentration of silicon...perhaps the latter, being a hot/volcanic location. That's a guess!
Just shorten it to pultravulcanosis.
That's what I always do.
that is the longest word in the english language Im told
Pseudopneumonoultramicroscopicsilicovolcanoconiosis? Slightly longer.
Separate names with a comma.