Can a decoupling capacitor be too large?
The type is more important than the value- if it's a smallish (eg. 0805 or smaller) surface-mount ceramic part, there is no disadvantage to a larger value capacitor. 

Compare the below two similar 0603 X7R Murata capacitors (top one is 1uF bottom is 100nF):



If you look at some reasonable impedance such as 1 ohm, the 1uF is &lt; 1 ohm for 250kHz to 600MHz and the 100nF from about 1.8MHz to 400MHz, so the 1uF is better everywhere (a decent regulator will fill in the lower frequencies, and a sluggish chip like the ATtiny won't create any edges with higher frequency content to worry about) so either is likely fine. 

You need to go onto the cap manufacturer's website and either download software or use web-based programs to get the actual behavior, it is usually omitted from datasheets in its full glory because there are too many possibilities. Note that the capacitance of the 1uF will actually be less because of the bias voltage which I didn't bother to set (it's just an example) but you should.