What size conduit (EMT) do I need for 1/0-1/0-1/0-2 cable (SER) for a subpanel?
If you're just using the conduit as a protective sleeve, and it's not a complete conduit system, I believe you can use the 100% fill volume of the conduit.  When putting cables in conduit, you calculate the fill based on the major diameter of the cable. This value can be found in the manufacturer's documentation, or by measuring the cable.  For Southwire 1/0-1/0-1/0-2 AL SER cable, the manufacturer lists the O.D. as 1168 mils (1.168"). To find the area of the cable, you'll just plug that value in to the following formula:

A = pi * (0.5 * O.D.)^2
A = pi * (0.584)^2
A = pi * 0.341056
A = 1.071

For EMT at 100% fill, 1 1/4" has a volume of 1.496 in.², which is large enough for the cable. However, if you don't have pulling equipment and lube, you may want to use a bit larger conduit.  

If it's considered a complete conduit system, you'll have to use the 53% fill volume. Which means you'll have to use at least 1 1/2" EMT (1.079 in.²).  Again, you'll likely want to use a size larger to make the pull easier.



NOTES:
- Conduit dimensions and percent area values can be found in Chapter 9 Table 4 of the National Electrical Code