According to Ruggles' book, "the restriction in the idle tubes varies greatly in different applications", so size does matter. However, it is also matched to the other idle passages, so just changing to larger tubes may not be a good idea.
The trapped dirt is under the idle tubes, so once you get them out it should be easy to clean out the area. You can also use a stiff wire to break loose any gunk that is trapped at the bottom.
If you got the tubes out, that is the hard part. You have to use a small drill bit or punch to drive the idle tube down, and then use a drywall screw to engage the outer collar and pull it out. Once the collar is out, the tube should just drop out when you turn the carb over.
To put in the new tubes I just pushed the tube and collar as far down as it would go, and then used a small piece of wood to drive it down until it was level with the carb. Take it slow, because you want the tube straight and you don't want to flare the top end by hitting it too hard.