I buy the 4 x 6 ones at Staples and I like them that size because I'm able to make a standard 4 x 6 photo printed on paper from my home printer that fits the card exactly...and they give me a little more room to write. But you may not need the photo at all unless you are recording seedlings...it is a handy reference to jog your memory of what the seedling looked like. The heavier card stock is more indestructible in the garden than notebooks with paper. I've left them out overnight in heavy dew, accidentally spilled coffee on them, etc and have genetic records at my fingertips going back to the 1990's without taking up too much space. I know there is probably a more modern electronic way to record information, but the garden can be a muddy, wet place that isn't good for electronics. I have a pretty organized "system"....once measurements have been taken and recorded, I'll cut or pull the spent scapes (except the ones with seedpods) so I always know which ones have been measured.
