We have 4 in ours and a 170L solar tank has proved just fine though the textbooks would say it is a little on the small side fora household of 4, though in our case we allow the solar to heat the water above 60c and then have thermostatic mixers at the sinks to eliminate the risk of anyone getting scalded. This way a smaller tank can store more energy for its size.
There are various rules of thumb one hears - One rule of thumb for tank sizing is 50L per person in the household, but that assumes you can put any size of panel array on your roof. If funds or available roof space for panels or indeed a small hotpress limits what you can do then you generally start with the thing you are limited on and work back from that.
Whilst flat panels are very commonplace in Ireland, it is far from uncommon to see a flat panel installation with condensation issues. Commercial installers like flat panels because they are generally cheaper but in my view evacuated tube type panels will give the least hassle and provide hot water for a longer season than flat panels.
If you want to give a little further detail on your plans/thoughts we are always happy to advise.