Forgive the massive necro here. You actually want iostat -x which will display the extended stats for the device in question since last iostat was run. If you wish to monitor the queue in realtime you want iostat -xt 1 (or iostat -xmt 1 to show details in megabytes).
You can see the average queue size in the avgqu-sz column.
Consider the following example output from iostat -xmt 1 which shows a full IO queue (max queue length is 128 for this device) and a saturated disk during a benchmark.
18/05/15 00:41:05
avg-cpu: %user %nice %system %iowait %steal %idle
0.00 0.00 0.00 6.02 0.00 93.98
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
xvda 0.00 0.00 1.00 1308.00 0.00 163.50 255.81 133.30 101.15 0.76 100.00
