Hi everyone and thanks for your replies. I'm getting more and more confused by the differences in lenght of similar snowboard models from different manufacturers. For example, when comparing the Rome Agent to the Burton Custom, which was supposed to be almost identical board, one can see that the latter is available in many different lenghts, going up to 169, while the longest Agent comes in 160. Bataleon Evil Twin and Riot also come longest in 159. Is that because Burton makes boards for a wider variety of riders weights, or is the Custom ridden longer due to differences in shape? For me, carving is currently the first thing I would like to learn properly. That' why I chose to stay with a cambered board. On the other hand, where I live the possibilities of riding powder are limited on only a few days per season, but there is park. Thats why I would go shorter - to allow for better park riding possibilities as I get better. So i thought that the Agent in 158 would be a good compromise for me... Would you recommend any other brand/model for me?
Sorry, trying to get back on topic. To OP, the reason why the Custom have more size options is simply a commercial decision - Burton makes/sell a LOT of snowboards (40% of the entire market I think) and so it can afford to have a lot of size options to fit it's massive customer base. I'm not sure, but I wouldn't be surprised that Burton makes more instances of the Burton Custom model in a year than Rome's entire production lineup since Rome has less than 2% of the market.
If there is no powder, then you probably can get away with going a few cm shorter. As for "park riding possibilities as you get better"... it is my opinion that you should get the board that is going to be good for you right now
instead of trying to compromise for something you might do far in the future. It's like buying an SUV because you might go off-roading (where in reality I'm pretty sure 99% of people never get rougher than a gravel road if even).
Having a board that is 2-3 cm longer is going to be barely noticeable when you are starting out in the park. Straight airs, grabs, 180s, 50-50 on rails, boardslides on rail all can easily be done by anyone who knows what they are doing on any reasonably length board of reasonable stiffness (i.e. unless you are getting the longest, stiffess board possible... you can learn park with any board). Once you are starting to attempt 540s and 270-to-boardslides, then because that's what you are doing a lot... getting a shorter, freestyle oriented board makes sense (my opinion... since I can actually spin a 540 and do a 270-to-board... I feel like my opinion is worth more than people who can't do those park tricks... but still try to recommend boards for general riding like they can).
If you are new to snowboard and not a teenager who rides park all day... realistically you are not likely going to improve fast enough to worry about needing a freestyle specific board while you own your current board.