I provided a number of blip textures to addon authors about 18 months ago. Since then, I've used a different set myself. I've attached them to this post in a zip file in case anyone would like to use them.
I've reordered the blips yet again since Blizzard has accidentally moved the PlayerPet blip to the 16th position, yet left its icon in the previous 15th position.
These blips should work reasonably well for minimap scales from 50% up to 400%. Here's a preview: (note that the blips will not appear this large in WoW unless you scale the minimap up to about 400%).
The solid squares are there as a reminder that those icons are currently unused. If you happen to see those solid squares appear on your minimap, please let me know what they represent.
The blips for humanoid and NPC tracking are actually about half as big in this version as the ones I created last year.
I like to keep the quest and resource tracking icons as large as possible because those are the ones that might otherwise get lost in a swarm of humanoid or NPC dots. The order in which blips get drawn on the minimap is not completely consistent. If the quest and resource blips were always the last ones to be drawn, on top of the others, I'd be comfortable making them smaller.
This is just my own personal preference, however. Feel free to take these and resize them or change them or use them however you want. Since there's very little artistic or technical merit to this particular set of shapes and colors, I consider them to be public domain. :)