The ratio will be dependent on the recipe since agave is a wet ingredient and sugar is dry - so you'll have to look at the ratio of wet to dry in toto. But in general terms "they" say agave is about 2x as sweet as sugar. In my personal experience I'd say it's less than 2x as sweet but sweeter nonetheless. So by weight you might start out with 1/2 the amount and adjust to taste. But that will be somewhat dependent on your other wet ingredients. Agave is primarily fructose and bakes very similar to high fructose corn syrup so it might help to think about it that way.
Slightly off topic: I don't want to preach but this is something I mention to everyone who is considering agave as an alternative for health reasons. In the interest of health concerns I'm just throwing this out there for consideration: My experimentation with alternative sweeteners started a couple years ago when found out my father was diabetic. After doing a little research I personally believe agave to be much less of a health alternative than is generally believed. It does have a low GI and it is diabetic friendly in that respect. But there's no free lunch - the high calorie/low GI means that the fructose is going to get transformed to triglycerides just like HFCS only more so. That's bad. Considering agave averages around 70% fructose that's very bad. Low GI = good but put in context not as good as it might first appear. Higher triglycerides are ironically associated with insulin resistance. If you're diabetic, you might be reaping the immediate benefits of a low GI sweet but losing in the long term with even lower insulin sensitivity. This is only true, of course, if you use agave regularly. If you're only using this as an occasional treat then disregard everything I just said. But still, it's not the product that it's advertised to be.
Ok, that's my little diatribe. In terms of quality, the best low GI sweetener that I've found is a combination sweetener called "Lakanto". It's expensive but actually registers zero on the GI scale and close to zero calories and bakes just like sugar. It contains some erythritol and so has a slightly "cool" taste when eaten directly. It doesn't have the bitterness or baking problems associated with Stevia or it's derivatives either. If you can get past the sticker shock, it's quite good.
Best of luck,