As with so much in life, size doesn't really matter. With addons, what matters more is
1) Are the addons good quality, commonly used, easy to find and so on?
The answer here I suspect is yes: most people have BAS stuff so if you are calculating how much folk will have to download you could almost exclude it from the calculation. 28 and 56kers are not going to touch a campaign like this anyway, (unless they are addon fanatics) so you don't really need to worry about them.
IMHO 50meg is not a problem per se. nominesine is right, you're as concerned with the number of addons as the total size. For me, that list would be fine for a campaign. (I've seen much longer lists for a single mission.)
2) Are the addons used well?
Nobody likes to download something and then not see much of it. Don't overuse, but I would say in a campaign you should strongly consider using each addon at least twice.
And don't forget that mission design is fundamentally about having fun mission designing. Sure its nice to get a good score and lots of downloads, but that's pretty much worthless if you didn't enjoy making the mission in the first place. Your primary concern is to make the campaign that you want to make: things like addon size are important, but ultimately secondary.