Settings for re-encoding a BD (.mkv) to MP4 (file size reduction)

Hi guys, I tried to search some guides on the web but I could find any specific about what I was looking for.

I've very little knowledge about encoding and I usually use Megui for encoding. Sometime I use Handbrake too. [Not an expert with neither of them]

Now I have this large .mkv file, 1.2 GB (1920x1080) with subtitles and I want to resize it to a 400 MB MP4 file (keeping the subs).

If I am not wrong, higher the bitrate is, bigger the file it is.

So, how should I proceed?

I heard about the command-line encode but it seems pretty hard and I'd not know where to begin with.

Thanks in advance for your help.

P.s. Sorry for my bad english.