Desma others have mentioned you can size pictures in Paint (if you use Windows). Also, I think you can open pictures in other applications, use the image editor there then resave to your picture folder. Another alternative is opening a free Photobucket account, upload photos there and edit as required. You can either post picture direct from Photobucket by posting the URL image code using the Insert Picture icon in the post editor OR save the edited/resized pic in photobucket back into your computer files under a new name.
Edit: Is this for pictures in the forums or adding pictures into you Profile album? For pictures in the forums, if they upload ok as an attachment (i.e fit size limit) you can insert the image in the post, click on the image and resize it using your mouse.