Welcome to the site !!
Beginner rider is one thing , but your size creates another issue ........
Do you have any bike experience at all ? , dirt ? , street ? , mini bikes ? , scooters ?? (like riding someone else's bike ??)
Due to your size , fitment is going to be a issue , what you are sized for , is a 450 , obviously that is out of the question for a newbie , but if you are just planning on casual riding trails , fire roads , etc. , then maybe a 250X or L would suit you better ** not a RX but a X**.... if you are not getting a R model , then I think you should be able to control the power , and so to speak , "grow into it" and become comfortable on it and not "outgrow" it quickly , both physically and skill wise , you will need to respring anything under a 450 , due to your weight
A 125 is for someone around 125-140 lbs. , a 250 for 140 - 165 lbs. , and a 450 for 165 - 185 lbs. optimally , so you would need stiffer springs , mainly shock , but depending on the actual bike , you would most likely need fork springs as well , for example , I weigh 203 on average (without riding gear) , I run a 5.8 shock spring and 0.47 fork springs , I am a little light on the fork because I have a really severe wrist injury , so I need it soft to save my wrist , I usually run a 0.48 fork spring ...... that said , a 450R/X is sprung with 5.5 shock /0.46 fork , a 250X is 0.42/4.8 , a 250R is 0.45/5.2 , all give or take a rate due to bike year , etc.
So comfort wise , you will "fit" the 250 better , and learning throttle control will keep you off the ground , or you could set up some kind of throttle limiter either in the carb or at the throttle tube itself if necessary , but I do not think you would need to ....
So I would recommend a 250 , the model is up to your budget and comfort level of confidence and the actual type of riding you will mainly be doing ......I will say the newer R models are a handful , so a X or a L would be more suited to a beginner with the ability to grow into the power usage and still ride with come degree of physical comfort , which is very important as far as fatigue and riding confidence ...... NOT the RX which is pretty much a R with slightly plusher suspension and a kickstand , they did stop making the 250X , but hopefully the dealer you work at has some leftovers you could get ....... I know Hillsboro ,Oregon (Motorsport Hillsboro)has lots of leftover models back to even 2017 brand new ready to go ......... Hope that helped ....
