There's at least a couple ways to figure out lengths in gameunits:
- Just pick an appropriate gridsize. E.g. if I set grid size to 32 and I move a brush up two increments, it will have gone up 64 units.
- To know the dimensions of brushes you have selected, just turn on size info. edit>preferences>2d>display size info (I think, can't double check right now).
There's probably a couple other ways, but I never use them.
edit: for your second question, you need to use the particle system.
Here is a detailed explanation of all the parameters you can use. Your particle system belongs in a mapname.particle file that goes in your /scripts directory. To add it to a map, use misc_particle_system. However, if you're new to mapping, it would probably be much, much easier for you to copy a particle system from another map.