In lieu of this, some photo sites like Flickr have options where you can choose what size photo you want to "Share". They may all have this option, to tell the truth I've never looked. I like to reduce the photo size first to save on upload time. But if you can't adjust the photo size on your computer, it's nice to know you can do it on the photo site.
However you bring the photo to this forum, it's always best to choose the "Preview" button to check out your post before you actually "Post" it. This gives you an idea what your post will look like before committing it to public view, and gives you an option to go back and make changes like choosing a different size from the "Share" option.
Good luck, and we look forward to viewing your photos.