spoons from baby and demitasse to serving spoons are totally dependent on the maker or manufacturer. There are NO standard sizes. Measuring equipment such as spoons and cups are to exact standards. Go buy a set or two. They are not expensive and make your cooking, baking and following recipes much more likely to work correctly rather than the total randomness of dinnerware.
This will be a pain but it will give you an accurate measure...one exact cup is 16 tablespoons. So half a cup is 8 and a quarter cup is 4. If you have a set of measuring cups, just use the quarter cup and see how many of your spoons it takes to fill it level...then you will be able to figure out how many to use in your recipe.
1 tablespoon should be very close to a measuring tablespoon, industry regulations should insure a similar size