No, you can't: that's for your "personal item". It's big enough for a small backpack with a laptop and a few other things, and that's about it. Certainly not for a couple days worth of clothes and toiletries.
I'm going to disagree with you there. I took a lot of trips on Ryanair recently, and since they charge for overhead bin use, I kept everything under the seat in front of me. I was able to fit everything I needed (toiletries, clothes, laptop) for four days in a backpack under the seat in front of me.
Ok fine, I suppose you can somehow make it fit if you pack only the bare essentials. I don't feel like it though: why should I? I don't fly RyanAir and I can use the overhead bin for free, so that's what I'm going to do. I can usually bring all the luggage I need for my entire trip carry-on and fit it in the overhead bin and under the seat (two bags, one large, one small), so that's what I do.
It's big enough - I've put my backpack filled with a weeks clothes for BOTH my husband and I plus my laptop under the seat in front of me.