Choose "ITRANS .itx input" format for text that is a complete .itx file. This will be processed using ITRANS and LaTeX. The {script-size} commands are not applicable for this format. Complete .itx files can be found in the doc folder of the files in the ITRANS download archive (available at the ITRANS home page).
The input ITRANS files can \input the following TeX files if needed: idevn.tex, and webincl.tex.
Note that the GIF image produced will show the first page only, but the PDF and PostScript files will contain all pages of your document.
Here's a example of one such document:
% --------------------------------------------------------------
\documentclass[12pt]{article}
% --------------------------------------------------------------
% Example ITRANS .itx input file for Online ITRANS Interface
% will be processed by LaTeX
% --------------------------------------------------------------
\input{webincl.tex} % this brings in many of the font size commands
%------ load in all font files for ITRANS
% (just an example, only those files needed can be loaded)
#teluguifm=tlgutx.ifm
#tamilifm=wntml.ifm
#kannadaifm=kantex.ifm
#bengaliifm=itxbeng.ifm
#gujaratiifm=itxguj.ifm
#hindiifm=dvng.ifm
#marathiifm=dvng.ifm
#sanskritifm=dvng.ifm
#indianifm=dvng.ifm
#gurmukhiifm=pun.ifm
#romanifm=romancsx.ifm
%----- all font commands - see webtml.tex for definitions
% (just an example, only those fonts needed can be defined)
#tamilfont=\itrfonttml
#telugufont=\itrfonttlg
#kannadafont=\itrfontkan
#bengalifont=\itrfontbng
#gujaratifont=\itrfontguj
#gurmukhifont=\itrfontgrm
#romanfont=\itrfontrcsx
#indianfont=\itrfontdvng
#sanskritfont=\itrfontdvng
#hindifont=\itrfontdvng
#marathifont=\itrfontdvng
% --- following sizes are available predefined
% \itrsizesmall \itrsizenormal \itrsizelarge \itrsizehuge
\begin{document}
% -------------------------------------------------------------------
\itrsizelarge
\section{Large size text}
#gujarati
bhaarata maaro desha chhe.
badhaa bhaaratIyo maaraa bhaa I baheno chhe.
huM maaraa deshane chaahuM chhuM ane tenaa samR^iddha ane
vaividhyapUrNa
vaarasaano mane garva chhe.
huM sadaaya tene laayaka banavaa prayatna karIsha.
#endgujarati
% -------------------------------------------------------------------
\itrsizenormal
\section{Normal size text}
#hindi
bhaarata maaro desha chhe.
badhaa bhaaratIyo maaraa bhaa I baheno chhe.
huM maaraa deshane chaahuM chhuM ane tenaa samR^iddha ane
vaividhyapUrNa
vaarasaano mane garva chhe.
huM sadaaya tene laayaka banavaa prayatna karIsha.
#endhindi
% -------------------------------------------------------------------
\itrsizehuge
\section{Huge size text}
#hindi
bhaarata maaro desha chhe.
badhaa bhaaratIyo maaraa bhaa I baheno chhe.
huM maaraa deshane chaahuM chhuM ane tenaa samR^iddha ane
vaividhyapUrNa
vaarasaano mane garva chhe.
huM sadaaya tene laayaka banavaa prayatna karIsha.
#endhindi
\end{document}
% -------------------------------------------------------------------
Your output should look like this:

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