Hi Pat,
We went there for a fortnight in August 2004 with two friends. Excellent, would go again but we still keep finding places we haven't been yet, but one day...
We stayed at a very large site called Bijela Uvala which is about 5km out of Porec on the Istrian peninsula. The site had about a couple of thousand pitches but was not at all overwhelming. We booked about this time of year (so about 10 months in advance) and I don't know whether that helped because we had a pair of fantastic pitches, only about 20 metres from the beach. There were only three british vans on the site when we were there - our two, and one owned by the then owners of "Select Sites" who were looking around for new sites on the periphery of their main operating area.
The facilities were fine, good shop, some activities for children, constantly cleaned service blocks. Highlight of the stay was a "fish picnic" on an all day cruise from Porec along the coast that was stunning value for money. However the financial world has moved on both here and in Croatia since then.
Here is a link I just found which may help -
http://www.alanrogers.com/camping/croatia/istria/CR6724
Do go, I am sure you will enjoy it.
Mike A