"She is looking for bigger pants."
Translation:Она ищет брюки побольше.
I'd say "штаны" is the more colloquial term.
Все это великолепие разбивалось о маленькую бумажку, прилепленную к входной двери магазина:
– Фу, как грубо, – сказал Остап, входя, – сразу видно, что провинция. Написала бы, как пишут в Москве: «Брюк нет», прилично и благородно. Граждане довольные расходятся по домам.
И. Ильф Е. Петров. Золотой теленок, 1931
All of this shattered magnificently when it bumped up against a small piece of paper affixed to the shop entrance:
"Ugh, how rude!" Ostap said as he went inside. "You can tell right off we're in the provinces. They could've written it decently and nobly, like they do in Moscow: 'No Trousers.' Then the citizens all go back home satisfied."
Ilya Ilf Evgeny Petrov. The Little Golden Calf, 1931
Is this like говорю по-русски?
No, that's completely unrelated. We add the prefix "по-" to the comparative form of an adjective when we imply something like "a bit", "just a little" before it. "Больше" - "bigger", "побольше" - "a little bigger", "страшнее" - "scarier", "пострашнее" - "a bit scarier", "сильнее" - "stronger", "посильнее" - "just a little stronger" and so on. Just like in English it doesn't really change the meaning, simply adds some flavor to it. However when we are talking about clothing size it's a common convention to use "побольше" and "поменьше" instead of "больше" and "меньше". Using the latter pair is not technically wrong, but it sounds awkward for a native speaker. Maybe that's because usually we are talking about a small difference when we ask for a different size in a store; maybe it's because "больше" and "меньше" are also adverbs, which respective meanings are "more"/"anymore" and "less", and people find that distracting; or maybe there's no reason at all and it is so simply because it is so.