If you get something small, like a ford transit based vehicle, you can fit into normal parking bays and can go just about anywhere in cities (but you may not want to) But not into multistorey carparks because of the height restrictions. During our one month in Europe that was never a problem. We did not go city exploring, but brought bikes for exploring countryside and campsites. Most shopping centres had a different layout to in the UK, where you build in height and park either on top or in basement of the centre. You got a big outdoors car park, and a shopping centre which was laid out lenghtwise with mostly just one or two stories.
I can totally recommend Poland for holidays, it is dirt cheap, food is lovely, and the campsites brilliant. Camping is very popular among Poles. We have been to the Northern Baltic Coast, to the Hel peninsula where my son did windsurfing lessons and we ate full blown restaurant meals in the campsite restaurant for less than £7 for the 4 of us. Mazury, or the lake district with great forests and lakes is another great area to go to, equally the southern mountain region.
Going again this summer, but as our budget is limited, we are flying to Warsaw, renting a car, and will go tenting instead....