The simple answer is - whatever gravel you can find that is the cheapest will be fine. There are a couple things about trenching for irrigation that you may also want to consider.

1. If you're only stubbing this out for future use, I would recommend visiting the following site - it's probably one of the best resources online for a layman's guide on how-to design and install an irrigation system...

http://www.irrigationtutorials.com/sprinkler00.htm
http://www.irrigationtutorials.com/install.htm

2. the depth of your mainline[1] needs to be at least 18" + (outer-diameter of your pipe). For a 2" (inner-diameter) SCH40 PVC pipe, that would be approximately 21" deep. Because this pipe is always charged with water, you want to make sure that it's out of reach from the errant stab from a tool (shovel, pickaxe, cast-iron tiki-torch, etc)

3. Once you're done with your trench, only fill it halfway at first and then flood it with water for a couple minutes. It'll help the gravel and soil settle around the pipes evenly and they'll be a lot less likely to break.

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**I'd honestly recommend that you hold off until you've done a little more research. Your mainline should be as short in length as possible while also being large enough in diameter to support the flow your system will require.

But you can't know how much flow you need until you've figured out where each head is going to go and which type of nozzle (360, 270, 180, 90, etc.) and distance for each pop-up as well as pressure and flow requirements for rotors.