Zenity allows some markup like <b>, <i> or <span>. In fact it's Pango Markup. So larger text can be achieved with a span that has a font or size attribute:
For example:
zenity --info --text '<span foreground="blue" font="32">Some\nbig text</span>\n\n<i>(it is also blue)</i>'
Note that font="32" is another way to write size="32768" since size is in 1024ths of a point. font="32" is a partial font description containing only a size. See the documentation page.
See also:
zenity --info --text 'Normal <big>Big <big>Bigger</big></big>'
To increase the font size without specifying an exact size.
--column="<span font='24'>logmsg</span>"
, and, unfortunately, for the lines to list, too. Maybe this is only configurable over system wide settings? – Qktp