Egg sizes of yesteryear and today I have a great Mrs Beetons recipe for Almond Icing.  However, as it's a very old recipe and calls for the whites of 4 eggs what size egg should I use.  As eggs on the whole have been getting smaller over the years.  Yes I can see that egg yolk sizes have been discussed on the site, but not this particular problem that I have.

ALMOND ICING FOR CAKES.


INGREDIENTS – To every lb. of finely-pounded loaf sugar allow 1 lb. of sweet almonds, the whites of 4 eggs, a little rose-water.


Mode.—Blanch the almonds, and pound them (a few at a time) in a mortar to a paste, adding a little rose-water to facilitate the operation. Whisk the whites of the eggs to a strong froth; mix them with the pounded almonds, stir in the sugar, and beat altogether. When the cake is sufficiently baked, lay on the almond icing, and put it into the oven to dry. Before laying this preparation on the cake, great care must be taken that it is nice and smooth, which is easily accomplished by well beating the mixture.
There is no absolute conversion as egg sizes were not standardized back then.  For the USA, not England, the food timeline http://www.foodtimeline.org/foodeggs.html under the section "Egg sizes the USA" (about 1/10th of the way down) starts with (bolding added):


  What size hen's egg was used to make a cake in the 1840s? Excellent question with no simple answer. Today's consumers find eggs clearly marked with quality grades in different sizes. In the 1840s, many consumers found eggs under the chicken in the family coop. Pre-industrial American cook books regularly acknowledge the relationship between fresh eggs and best product but are silent on size. Cook books focus on identifying "good" eggs, proper storage, and preservation techniques. In those days, the size of chicken eggs depended upon breed, feed, season and cooping conditions.


Recreating old recipes with modern products always poses special challenges. When eggs are on the shopping list, think small or medium.

The section continues with various extracts describing approximate weights and other commentary.

From the above, I would go with Dorothy's comment and use medium, starting with two or three and adjust for consistency.  Don't forget to weigh what you use so that you can repeat your success with any eggs.