The default is that the text width and the text height are 0.7 of the paper width and height. This means that
textwidth = 0.7 paperwidth = 1675.8pt
textheight = 0.7 paperheight = 4788pt
The left and right margins are, for one sided document, equal to each other; this gives
leftmargin = rightmargin = .15 paperwidth = 359.1pt
The vertical spaces are divided in a 2:3 proportion. So
topmargin = (2/5)(.3 paperheight) = 820.8pt
bottommargin = (3/5)(.3 paperheight) = 1231.2pt
See p. 13 of the manual (section 6.4).
The actual results are shown in the log file:
* driver: pdftex
* paper: custom
* layout: <same size as paper>
* layoutoffset:(h,v)=(0.0pt,0.0pt)
* modes:
* h-part:(L,W,R)=(359.10365pt, 1675.7927pt, 359.10365pt)
* v-part:(T,H,B)=(820.80835pt, 4787.97913pt, 1231.21252pt)
* \paperwidth=2394.0pt
* \paperheight=6840.0pt
* \textwidth=1675.7927pt
* \textheight=4787.97913pt
* \oddsidemargin=286.83366pt
* \evensidemargin=286.83366pt
* \topmargin=711.53836pt
* \headheight=12.0pt
* \headsep=25.0pt
* \topskip=10.0pt
* \footskip=30.0pt
* \marginparwidth=65.0pt
* \marginparsep=11.0pt
* \columnsep=10.0pt
* \skip\footins=9.0pt plus 4.0pt minus 2.0pt
* \hoffset=0.0pt
* \voffset=0.0pt
* \mag=1000
* \@twocolumnfalse
* \@twosidefalse
* \@mparswitchfalse
* \@reversemarginfalse
* (1in=72.27pt=25.4mm, 1cm=28.453pt)
The small differences from the "exact" values are due to the way TeX performs computation (in integer arithmetic).