How do I build small or medium ship to go to Mars.

[MEGAMIND]

So everytime I use medium or small ship with high delta V ,it ended up needing a rescue. but on mars I haven't try it yet.

[GunsForBucks]

duna?

Check your DV for where you are going to be at each stage.
https://steamcommunity.com/sharedfiles/filedetails/?id=2715079353

Make sure to intercept properly for easy capture.
https://steamcommunity.com/sharedfiles/filedetails/?id=2731321701

[Jupiter3927]

Building small ships lets you take advantage of the tiny engines for larger amounts of dV.
Spark engines tend to beat out the Terrier on small ships, even though the ISP is lower.
Play with different engines and see which one works best for you.

[MEGAMIND]

Originally posted by Jupiter3927:

Building small ships lets you take advantage of the tiny engines for larger amounts of dV.
Spark engines tend to beat out the Terrier on small ships, even though the ISP is lower.
Play with different engines and see which one works best for you.

I saw a video where they go to mars with very small ship with 4000 m/s only and still reach mars and back

[Quizzical]

Here's my small vehicle to go land on Duna and then return to Kerbin cheaply. Or at least I assume that you mean Duna and not Mars, as you can't get to Mars from Kerbin:

https://steamcommunity.com/sharedfiles/filedetails/?id=2458034104

I suspect, however, that your bigger problem is not so much one of building a vehicle as figuring out what to do with it.