My
experience is growing hybrid poplar OP367.
The size of cutting doesn't matter as long as it contains at least two buds.
I put the cuttings vertical in water with the water level just below the top bud. You can leave them there till you see
roots or you can plant them in the ground if it's not too hot and dry where you are. Plant with the top bud just above ground level and make sure the soil doesn't dry out.
You could go for a misting system and/or use an air pump to aerate the water while the cuttings are soaking. It isn't necessary but will give faster rooting.
Let us know how you get on and post some pics.