tldr: you can learn with any stick that you practice with. between chest and chin height is 'best' for a full size contact staff. diameter affects roll, and comes down to personal preference unless youre quite small or quite large.

length
for a full sized contact staff, you generally want the length to be somewhere between your chest and chin, but if you havent watched world class baton twirlers, you should watch anetta lucero, and you will see that about everything we aspire to do with sticks can be done with quite short ones. the longer the stick, the slower it will spin, and the more leverage it will have available to get it to keep spinning. for example, horizontal propellers like in a 'matrix' will be easiest with a super long stick. the shorter the stick is, the more space you will have for moving it around you or moving around it without collisions. shorter sticks are easier to throw. if you're trying to choose between the 54 and 60 inch defaults i offer, and both are somewhere between chest and chin, i'd say go with the shorter if you have or want to develop a dancey style and have a little more space for the stick, and go with the longer if youre more interested in extended contact with little mobility. the difference in the six inches is not extreme, tho, and really, you can only determine your personal preferences by playing with multiple, different props.
diameter
besides how it fits your hand, the main difference caused by different diameters is the roll speed. a thinner stick will require more rotations to travel from point a to point b. thinner sticks can be a little more difficult tto get rolling, but once they are, can feel stickier. the thinner diameter may demand a little bit more control to complete a move, but will also give you a little bit more control in directing the move. again, this is subtle, and preference can really only be determined by trying the different things. if you're under 5'4", i usually recommend a 5/8" tube. if you're over 6' usually the 3/4", and i rarely reccomend 7/8" (especially with a thick grip) except for dragons, unless you're quite large, or really want a fast rolling staff, but there are no rules.