I would say three nights, four days is the absolute minimum. Two days for the
Disneyland Park and one day for Walt Disney Studios.
We go for a minimum of four nights, five days and we find that this is enough time to spend a relaxing time in the parks. We also try and avoid the weekends as the parks are busy on Saturday and Sunday.
We travel by direct Eurostar from London so we arrive around 2pm on our first day at the resort and depart around 4.30pm on our last day.
I find that four nights gives you a nice relaxing trip without the need to rush around.
Our last visit to DLP was in March for the St David's Welsh Festival which was held Friday - Sunday. So we went for six nights, seven days. Arriving on the Thursday and departing on the following Wednesday. Which was more than enough time to have a very relaxing trip.
The Disneyland Park in Paris is larger than the Magic Kingdom at Walt Disney World and Hong Kong Disneyland. One thing that DLP benefits from is that everything in the resort is walkable. The two parks are next door to each other so park hopping is really possible. All the hotels around Lake Disney are about 10 -15 mins walk through the Disney Village to the two parks. Even the furthest hotels the Santa Fe and Hotel Cheyenne are only a 20 mins walk away from the parks along the river.
Another good design feature of the resort is that the train station is located at the park entrances in the Disney Village and that in DL there are lots of covered walkways, which is very handy for our European weather. Gives you shade in summer and dry on cold, wet, snowy days in winter.
There is a very good interactive map of the resort on the DLP website
here.
Hope this helps.