First of all, I'm going to use the estimates from this question, they should be good enough for the number of wizards. This will remain somewhat in the background. Also, I'm only going to count human wizards, I'm not going to try and figure out the rest.
The best estimate for the number of inner circle death eaters in the first war comes from the resurrection scene in Goblet of Fire. When all of the death eaters return, there is a dialog of several pages, but I recall that there were about 12 missing, and about 30 or so present, giving the total number to be 40.
Assuming this number for the inner circle, we can speculate that there would be at least 5 outer circle for every inner, quite possibly more. So, the total number of death eaters should be between 250-300.
The best clue to the number of people fighting is the photo in Order of the Phoenix. There are 26 people who are in the photograph or mentioned as being original members of the Order. I think we can assume that there are at least 4 others for every one we know about who fought Voldemort, so maybe around 125. We know that the order was seriously outnumbered, as is stated at the end.
So, given my rough guessing, there were around 400-500 wizards actively participating in the war. Probably only 20% of the population of wizards of England. No doubt there were others who were neutral, trying to keep Muggles from finding Wizards and the like, but probably not all that many.