zilly: Fear not, Mask path nodes are always the same size (Although it's interesting to note how Adobe Illustrator recently got this feature in which path nodes and control points magnify to stress which one is on target for selection).
Going back to your description of the problem, it could be useful to ask: You mean just Mask nodes or you may also include Shape path nodes? Because Mask path nodes are always editable, whereas shape paths can be parametric (ie, not based on editable nodes but on parameters for a shape, like outer radius, number of points, corner roundness, etc) or path-based just like Mask paths are, which can trigger some confusion.
In any case, I suggest you to take a look at the Drawing, painting and paths chapter in general, and the Editing and animating shape paths and mask section in particular, in After Effects CS4 Help on the web. It's very likely that it covers the issues that are confusing for you in this regard. If it doesn't, please let me know.
PS: I just lost the whole of this message when I tried to post it, because of a connection issue. It was a relief to find that the system offered me an option to recover the lost message! Way to go!
What graphics card do you have and which screen resolution? What type of computer? Could simply be general lag. Also, are you working in the comp window or the layer window? Ifd you work in teh comp window and have additional effects applied, calculating those effects can considerably slow down things, therefore work in the layer window and choose not to preview any effects results. In addition, consider, that oftentimes when you move to a new position in the timeline, all vertices of a mask will be selected instead of unselected. This odd behavior has plagued artists for a long time and you have to click outside the mask to deselct them all.
Whoops, I just saw that I had locked the mask. I unlocked it and suddenly it's editable. Still, sometimes a box appears around the edges of the mask and I have to get it to disappear before I can edit, and sometimes I can't edit when there's no box. What I meant about the vertices being small is that when they look like an empty box I can edit them, and when they're solid I can't. I made the mask myself, so I assume it has mask nodes rather than another kind.
I'm editing in the comp window, since the mask is to reveal something in the layer below. My computer isn't powerful, but I think it should be OK for CS3. I'm using Vista Home 64 bit with 4GB ram. Some Nvidia card that came with the HP computer. Screen resolution 1920x1200.
Indeed! Locking masks would complicate matetrs considerably! *giggle* ;-) The mask vertices being drawn as hollow boxes/ circles while inactive is correct, but not particularly visual. I can agree on that, but it's been like that for eons... The bounding box drawing is probably related to selecting the layer with teh locked mask - AE will use only opaque areas to determine the "layer size", which of course is less than the actual dimensions with masks applied... Again, perfectly normal, even if a bit confusing.