A few pieces of advice, most of which seem to have already been covered:
1) Dress on the formal side, unless it's inappropriate in your work environment. A suit and tie, especially a three-piece suit, will add a bit of gravitas. Caveat: If your work environment is super casual, this will make you look ridiculous, so consider the circumstances.
2) Make sure your suits and dress shirts fit well. Off the rack clothing is fine if it fits and looks good. If you're relatively slim, lean towards slimmer cuts and definitely avoid anything baggy. In my opinion a cut-away collar tends to work well with slimmer men (I'm one).
3) Try to put on some additional muscle, especially in your shoulders and chest. This will help fill out a suit and improve your visual impression.
4) Try to improve your posture. I think this is actually more important than your physical size. Sitting up straight and standing up straight help a lot.
5) In meetings, avoid talking too quickly or raising the pitch of your voice. A lower voice carries more authority.
6) If relevant, acknowledge your lack of experience before making a point in meetings. This may help defuse the issue in advance and mitigate some of the resistance from your coworkers. Try to be conscious of how you might appear to a man in his 50s. You're 26 and you're in meetings with 50-year-olds. Imagine what it would be like if you had to meet with someone half your age, and that 13-year-old insisted on giving you advice. Your ideas may be great, but consider that there's going to be some natural resistance and approach that aspect of the relationship with empathy. In my experience, this sort of approach is likely to make you more effective.