Modelman, I think I can answer your actual question as you posed it.
You can tell what size they will by what size you upload them. So whatever size you make them before you upload them, that's the size they will appear.
Photos for display on computers are commonly measured in pixels -- meaning dots.
Photos uploaded here should be 800 pixels across or less. I commonly use 640 or 480 across, very occasionally 800 if something really needs to be blown up to see details.
If you try to upload pictures wider than 800 pixels the whole picture will not be seen and users will have to scroll across to see all the parts of the picture.
Since most cameras take pictures thousands of pixels across, you need to shrink them down to a more manageable size before uploading them to this forum.
The way you do that is by using a photo program on your computer. All such programs have a feature to "scale" a photo. Often you have a choice to scale by inches, mm, or pixels. Choose pixels and then scale them down to the size you want.
There are a huge number of photo programs, and everyone has a favorite. It doesn't matter which one you use, just get familiar with this one scaling feature, and you're good to go!
Hope that helps!
