If you need large zoom by default, you can set it up in Preferences -> Page display -> Default zoom, but this would zoom ALL page not only text.
Else, look
this thread for needed solution (macro Autozoom)
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Using
K-Meleon-1.6.17db+ /
KM-16-S2014
1.6beta2.5 by JamesD
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Recommending K-Meleon Twin+
Try and
discuss it!!!