i don't know where to post it and i post this topic here cuz i need some help!how can i resize a gif image ?can anyone tell me???

if yer in photoshop..
it's the drop down menu under "Image"... choose "size"

do you mean resize as in optimise it to get it inside the required limit ?

yes!!!!!!!

Fitting into the size restrictions - open the Optimize palette (Window - Optimize), open the optimized view (tab) of your image and look at the information bar under the image. The default setting in the right-hand information field is Original/Optimized File Size. Experiment with the Color Table and Dither settings (i.e. Adaptive 128 colors / Diffusion 85%) and monitor the file size in the Optimized view. Run play to see how it affects the quality of the gif. Play some more to fit in the restrictions.

Worse case scenario - you may need to resize/crop the pixel dimensions on the GIF (e.g. to 412x248) and change the canvas size to the required 550x350 and go through the steps above again - the resulting white frame around it will blend in the site background.