FFmpeg has three concatenation methods:

1. concat video filter

Use this method if your inputs do not have the same parameters (width, height, etc), or are not the same formats/codecs, or if you want to perform any filtering.

Note that this method performs a re-encode of all inputs. If you want to avoid the re-encode, you could re-encode just the inputs that don't match so they share the same codec and other parameters, then use the concat demuxer to avoid re-encoding everything.

ffmpeg -i opening.mkv -i episode.mkv -i ending.mkv \
-filter_complex "[0:v] [0:a] [1:v] [1:a] [2:v] [2:a] \
concat=n=3:v=1:a=1 [v] [a]" \
-map "[v]" -map "[a]" output.mkv

2. concat demuxer

Use this method when you want to avoid a re-encode and your format does not support file-level concatenation (most files used by general users do not support file-level concatenation).

$ cat mylist.txt
file '/path/to/file1'
file '/path/to/file2'
file '/path/to/file3'
    
$ ffmpeg -f concat -safe 0 -i mylist.txt -c copy output.mp4

For Windows:

(echo file 'first file.mp4' & echo file 'second file.mp4' )>list.txt
ffmpeg -safe 0 -f concat -i list.txt -c copy output.mp4

3. concat protocol

Use this method with formats that support file-level concatenation (MPEG-1, MPEG-2 PS, DV). Do not use with MP4.

ffmpeg -i "concat:input1|input2" -codec copy output.mkv

This method does not work for many formats, including MP4, due to the nature of these formats and the simplistic concatenation performed by this method.

If in doubt about which method to use, try the concat demuxer.

Also see

• FFmpeg FAQ: How can I join video files?
• FFmpeg Wiki: How to concatenate (join, merge) media files

2592 by 2160 is ~20MB per frame uncompressed. A bit rate of 1200 bits/second will be way too low.

To take your enumerated questions in turn:

1. Dimensions of video - are there any limitations?

FFmpeg has no particular limitation that I'm aware of, but different codec and container formats do often limit the frame size and rate (or the bitrate which implicitly limits the latter two). You should check the specs for the codec and container format you're using.

2. Frame rate - can ffmpeg pad out videos?

As you note and mentioned above, codecs and containers don't always allow arbitrary frame rates. (And playback software may not work correctly even if the codec/container does let you set a crazy frame rate.) FFmpeg will pad videos for you, though. From the docs:

-r[:stream_specifier] fps (input/output,per-stream)
As an output option, duplicate or drop input frames to achieve constant output frame rate fps.

3. File size - are there any limitations on the max file size that can be converted?

Not that I'm aware of.