You don't really have to do this. Determining the dimensions of your table is simple geometery. You match image dimensions with that specific cell's dimensions, then you create neighbor cells next to the image table to contain your text or other images. This way you can fully control your table elements. I usually do not put images and text in the same cell because different browsers calculate image and text placement different. This is the old way of doing web pages I might warn you but it is probably the easiest for beginners. If you really want control you should learn CSS like Roshi229 mentioned. Good luck.
P.S. If for some crazy reason you still want to determine the tables dimensions after automatic placement, you could view it in your web browser and press the "Print Screen" button to take a screen shot. Then you could paste it in an image browser and crop the table out of the screen shot. Depending on what program you are using you can check the dimensions of the new image. If you can't do this in your software, save the file and view it in explorer. That should tell you the dimensions.