What were the relative sizes of Land, Sea and Air Forces in WW1?
Army
Infantry, artillery and cavalry: http://bjmh.org.uk/index.php/bjmh/article/view/96 
Navy
http://bjmh.org.uk/index.php/bjmh/article/view/165 
https://www.wereldoorlog1418.nl/RND-Royal-Naval-Division/index.html
Airforce By 1918 the Royal Flying Corps comprised around 400,000 and the Royal Naval Air Services some 55,000. https://www.gatewaysfww.org.uk/blog/RAF-100;
http://bjmh.org.uk/index.php/bjmh/article/view/209; http://bjmh.org.uk/index.php/bjmh/article/view/213