I am going on a week hike in Southern Spain - I'm intending on packing as light as possible, but will be taking lightweight mat, sleeping bag, bivvy or tent, stove, mess tin, minimal cloths, water, camera, notepad. I'm thinking about a 50L lightweight pack. Does this sound reasonable?
Although I agree 35L can be enough if you only intend to carry two changes of clothes and some small gear, I'm not sure you'll be comfortable enough having most of your things hanging outside your backpack (and this will be the case for at least your tent, your sleeping mat -either on top or bottom of your bag-, and maybe your stove and tins. Even your shoes, if it's summer).
You can put your camera, your first aid kit (bring one!) and your clothes inside. You'll still be fine with a 35L, but if you are also bringing food and water you might consider a 50L. It really depends on what sort of trip you are planning, what places you want to visit and what's available around those. You'll still have to carry whatever you chose to bring, but backpacks don't have much weight by themselves so a bigger one is not that costly.
When I hiked the Grand Canyon, I just used a 35L pack - the kind kids take to school. The smaller the back, the better the hike.
If you are taking a bivvy, two changes of clothes and the gear you mentioned, you might be surprised to find that it is all you need. - indeed, the biggest thing you are talking about - the sleeping mat - probably won't fit inside of either pack, so it's not really a consideration.
My suggestion is to try both. If you can fit all you need in a 35L, use that. If you really need the gear, go for it.
In the end, any pack - be it a backpack or a suitcase - is just a case in which you choose to place what you need. If you can live within the parameters it sets, it's good.