I blame fat cursors for this so I use the DMZ cursor theme. Still, the issue you mention seems to be generic to linux as I have exactly the same problem in KDE. The corner grips are simply not big enough and it takes a lot of mouse manoeuvering to be able to grab them. Sometimes I give up in frustration and resize windows using the horizontal and vertical grips independently.
That said, it's more likely due to the mouse being set to too high a resolution. I have one of those new-fangled Microsoft Sidewinder mice that has on-the-fly adjustable resolution. Dropping the mouse to a lower resolution results in success. You might look at your mouse settings and adjust the resolution down a bit so that it takes a bit more mouse movement to cover a given distance.
Mint Testing Team & Mint Donor #3606
KDE 4.11.3, custom preemptive kernel 3.12.2,
Intel i7 4770K @ 4.7GHz, 16GB 2666MHz XMP,
4 Samsung 840 PRO 512GB SSDs in RAID0,
6TB HW RAID10, dual 24" Acer X243H,
Gigabyte nVidia GTX 680 Super Overclock