Below you can see the front view size comparison of Nikon D90 and Nikon D300. Nikon D90 is 15mm narrower and 11mm shorter than Nikon D300 but it is also 3mm thicker.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Nikon D90 is significantly lighter (222g ) than the Nikon D300 which may become a big advantage especially on long walking trips.
Also keep in mind that body weight is not the only deciding factor when comparing two interchangeable camera bodies, you have to also take into account the lenses that you will be using with these bodies. Since both Nikon D90 and Nikon D300 have the same APS-C sized sensor, their lenses for a similar focal length and aperture will be similar in size and weight.