I don't know how to answer this exactly but in my previous outlook - There is mix genes of Polish now - not all are clean Slavic.
I believe that slavic peoples originally were of between light/dark/. Hair color between dark blond to dark brown. Fair - olive skinned. Any color eyes, actually I often hear of green eyes (but not sure about it).
In Poland you can find many people who are darker looking but had blond hair as kids and dont even look similar to themselves in comparison.
Also, these days in Poland (and for a few centuries now) there have been the appearance of the red-haired and very fair skinned peoples (often with appearance of frekled skin).
I think lots of Polish people range in the hair colors of dark blond - dark brown. Any color eyes (But often I see, green, gray and brown). Skin color tones also range from fair to olive. Both tones are the ones that tan well - Lots of Polish (especially girls) like to tan and be darker (not all obviously).
Height wise?? I don't know - I think average is something like 160-165cm? There are a lot of girls who are "tall" but even girls 168/170cm + are being labelled as tall.
Build wise - well, cant speak about older peoples as it is a different matter but in age 30 and less I would say a normal slim - a range of slim-normal/average buld. I have not seen so many overweight people in Poland (it doesn happen but not as many as I saw in US and Australia). But also I dont see as many annorexic looking people in Poland(as I also saw in US and Australia in contrast to the overweight).
On the mix of genes remember most Polish are not pure slavic. Slavic peoples were not the only ethnicity that created the state of Poland, although it was the majority and people did not mix as much then as they do now - there was also tribles are Germanic (which I believe are lighter in ethnic look *typically*). Also later on the introduction of nomadic tribes as gypsies (Romani), who didnt not generally mix but will find that most People will not own up to the truths of their families sharing Romani genes. Also not to forget the Jewish who came to Poland.
Hope to also hear responses from others I'm also interested to learn more on this topic! I am also little mixed in ethicity. But as a Polish I am proud Kashubian hehehe :-)
Now onto the features I notice thats its either a very sqaure face with big, beautiful, high cheekbones, or an slender heart face with the very pointy high cheekbones. The eyes shape tends to be lovley almond shape, and the lips full and unusual shape the upper lip bigger than the lower lip. Which nice gives it distinction. The nose is either many times nice defintley high bridged nose with globular round tip and rather large nostrils or a small very, very thin, medium, to high bridged nose with narrow nostrils.
This is pretty perfect description of my observation. Many many of the guys seem to have more of the square face shape than the girls. A lot of Polish guys share a more similar look than girls in my opinion also....