I am looking for the largets possible screen that you can buy, when
the device needs to run windows mobile 5.0. I have seen devices like
the ipaq 6915, but with a screensize of 3" it is too small. If
possible i would like to go up to 7". Keyboard is not nessesary, only
touch-screen is. What are my options. It is very important that the
device can run windows mobile 5.0 and that you can place a simcard
into the device.

Hi 

There are a number of devices with larger than 3inch (77mm) screens.
However you didn't specify if you wanted a phone built in or not.

PDA Only
Tosheba e830 (2004)4" 
The Dell AximTM  X51 3.7" around £314GBP
DualCor Technologies cPC say they have a 5" but I couldn't find anyone
selling this device.

PDA inc Phone
The HP has the largest screen I could find matching your criteria.
However the 2003 XDA II has a 3.5" screen but a lower O/S.

Just for fun. 
Maxi Handy (Maxi Mobile) made a 2.05 x 0.83 x 0.45 m
(6.72 x 2.72 x 1.47 ft) screen but doesn't run the correct O/S

The largest PDA phone that runs Windows Mobile 5.0 would be the HTC
Universal; also known as i-Mate JASJAR, QTEK 9000, T-Mobile MDA Pro
and a few other names. I own an i-Mate JASJAR and I can assure you,
the screen is very large.

You won't really find any larger PDA phone as there isn't enough demand for them.