These are just guidelines, but I hope it helps:
- Mobile: around 320pts
- Tablet: Around 768pts (portrait)
- Browsers: 1024+ pts
There are a couple of notions behind this, so, to make sure I don't give these dimensions in a vacuum:
I measure them in pts. A Retina screen has different px dimensions for the same physical size, but it will have the same points measurements no matter the pixel density (used for positioning elements and sizing fonts). More on that here if you're interested. Having the right density is good for previewing on the device, but you have to make sure you don't lose track of the actual measurements that developers will use.
Mostly, you should use one of these dimensions and start your design and layout where you're comfortable. You can decide on breakpoints when that layout starts to break, instead of going with the dimensions first and then having to "shoehorn" layout changes into predefined breakpoints.
If you use artboards, I suggest doing everything at 1x dimensions (1px = 1pt), then doing the scaling to 2x and/or 3x in production.