airBaltic has not been strict / measured the hand baggage during last couple of years on my flights - and haven't seen this at RIX. However, they introduced the 12kg cabin baggage option maybe a month or two ago so not sure if this is going to change. There is no free seat selection at OLCI, they assign them for you. I haven't seen people changing seats after take off as much as on Ryanair for example, so assume they assign the seats together.