Yeah you'd probably be ok with them that long, just take the cutting immediately below a growing bud, trim off all the leaves bar the top 3, 4 or 5 and cut those in half. Depending on your soil, (should be nice and friable) they should take ok straight in the ground. I'd also use a little hormone powder or fluid on the cutting, after scraping a bit of bark back to expose the cambium tissue. A couple of years ago I decided on something similar and started a dozen cuttings, 6 to 10 inches long in pots. They all took and I planted them out but as I don't like oleander's habit of becoming a cluster of ground arising sticks I keep the stems clean of growth for around about the first 75mils to a metre - training them as single stem bushes. After a few years they tend to give up trying to sprout from along the trunk and produce not unattracting small trees. The pink especially in the warmer climes looks lovely when in full flower. In a group watch for scale insect though. I have it affecting 2 but not the 10 I have in. White oil is effective on that.

if you do them in pots first you have more control over the moisture but they do take root easily so they may work just stuck in the ground but its more hit and miss. Ive also had good success with them started in pots then its just a matter of planting them out in the garden where you want them.