My friend and I want to put space in our blog. We want to have ideas what sizes are common for ads space and how much shall we charge for it?
Wow that's too general and too broad. I can tell you about my blog: I sell 250x250 size banners on my blog. I know people who use smaller sizes: 125x125 is common too. Also, you can sell a top banner which is like 600x15 (i think, not too sure on this, check it) Price of course depends on the number of unique visitors your blog gets and also your niche. If advertisers find value in your blog, they can pay more. If you provide more details on your blog, perhaps it will help gauge it better.
250 x 250 150 x 150 300 x 200 Cost depends on how much visitors you have. If you got lots of traffic then you should charge them according to it. I've an idea for you, Visit buysellads.com and see at what ad blocks are selling according to the number of impressions.
^^ If you guys have a very popular website, then you can charge more. You'll want to prove to your customers how much traffic you can get/have. If you have a lot, you can use that to prove them they need to pay more to get the ad space on your site. Sizes are mostly preference to what you can fix on your site. If you want a lot of available space, then you need smaller (pixel)x(pixel) spaces which means less costs per one space. I would put a few of each (Boxes 150x150, Rectangles 300x200, Banners, Leaderboards, etc) and you can adjust sizes as well to fit your customers needs.