Standard fuse is 3A on the AL7, I wouldn't change from the OEM - really the fuse is to protect the wiring against shorts (I.e. the wiring chafes through, and shorts to the frame, the fuse will blow), rather than protecting the end device.
12V at 3A is 36W, a USB charger outputting 3A (1x2A and 1x1A) is only going to output about 15W, and high efficiency switching buck converters are used even in cheaper electronics now, so probably not more than 20W of power used at full whack.
One possible hurdle you may have is the connector if you don't wish to hack your OEM harness apart (if it's the same sort as the AL7). I can't recall what connector it was (I had mine off repairing the wiring from the previous owners accessories that they'd removed), but there was a short pigtail from the actual socket which joined a plug on the n/s/f harness.
Edit: Here's a pic of the pigtail end (the pin is half out because I removed the plug to pop some glue lined heatshrink on to repair the wires). Again AL7, but may be similar!