It is a single DIN that means AFAIK
that the face is 7 x 2 inches i.e. a "normal" car radios depth. The double DIN units have more features as they are 7 x 4 inches.
How far back into the dash they go is determined by the manufacturer, it may be that some units go in too far, the only way of checking that is to remove the old one and measure it. Then check a new one will fit - if it is VERY much deeper into the dash it may not go or may need extra support - some units come with extra straps to hold the back still INSIDE the dashboard aperture.
As regards what unit, there is such a vast selection that you may gins everyone has their own advice.
BUT think carefully, in the UK the best reception may well come from a DAB unit, whereas the same system will not work widely in the EU where alternatives may be found....
You may also find it beneficial to find a unit with a USB input as it is easy to download music on a USB stick and fit massive numbers on one stick c/w a single CD.....
The USB input can also be used for music stored on a phone, a Bluetooth connection may also be beneficial not only to input music, but also for mobile phone hands-free operation where the music may be paused if the phone is in use automatically.
So, the choice is as deep as your pocket..... but single DIN is the one to hold for in a 2003 MH UNLESS examination shows a double DIN unit to be fitted already.... 🙄