How to save a plot as jpeg or pdf with a given aspect ratio and resolution?

I need to save a plot generated by my matlab code as a jpeg or pdf file. The simple 'saveas' command works, but the resolution is poor, the aspect ratio of the generated jpeg is not the one I want (it seems to generate pictures only in a given aspect ratio) because of which the legend table and/or the axes get cut in the resulting picture file. Please help me with the codes that will save my plot as a jpeg or pdf with the resolution and aspect ratio specified in the program.