SiTime mentioned above is selling oscillators that can be embedded in the same package as the microcontroller. I would say that's the way to go.
I wonder how SiLabs compares to the competition? Personally, I tend to favor MEMS platforms based on SOI that are DRIE:d into the structures. Mostly due to the simplicity and the use of monocrystalline Si. SiLabs deposit a pretty thick layer of poly-SiGe (SiGe can be deposited at pretty low temperatures which is critical if CMOS-wafers are the substrate that is deposited) that is formed into the resonator structure.
Guess in the end it's a question about the yield of the processes.
The 2mm x 2.5mm size is nice. It's disappointing how large a lot of the current oscillators are compared to the tiny chips around them. Sometimes the oscillator is bigger than the MCU.
My next question is, when can I get these integrated with the MCU on the same dice?