The DR-Z 250 is an awesome bike. I had the 2007 model as my first ever motorcycle and never had any issues with it.
Answering your question about a bigger tank: back then I sent an email to the guys at Safari Tank. According to them their DR-Z400 long range tank would also fit the DR-Z250. Not sure though if that works for your model of the 250 too. Maybe worth getting in contact with them.
Also be aware that a long range tank adds a lot of weight mostly onto your forks which might need upgrading. That was my main reason for not going ahead with the tank swap.
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