Heatsink size for LEDs

[Jade5051]

I'm going to make my own LED fixture for my ATS. Wanted to use these (first link) heatsinks and place 4x3w LEDs on them. The dimensions are 6"x1"x0.25". Do I need larger heatsinks?

The 2nd link would be my alternative and I would place 8x3w LEDs on them. The dimensions are 6"x2.36"x1". Would that be ok?

http://www.ebay.com/itm/111807742941?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

http://www.ebay.com/itm/400918246072?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

 

[mattberrytr]

First link is unlikely to be enough for four LEDs.
Second link should be fine for 8 LEDs.