According to the Megillah (9:4), Mordechai grew to a very large size whenever he was in the king's palace (כִּי־גָדוֹל מָרְדֳּכַי בְּבֵית הַמֶּלֶךְ ... כִּי־הָאִישׁ מָרְדֳּכַי, הוֹלֵךְ וְגָדוֹל).
How large did Mordechai grow, and did he return to normal size upon venturing outside?
It is known that מרדכי's other name was "פתחיה". The names are related as follows: If you double the g'matriya of the first three letters of "מרדכי", you get the first three of "פתחיה"; and if you halve the last two of "מרדכי", you get the last two of "פתחיה".
Continuing the trend, let's multiply the letters of "מרדכי" by other powers of two:
Thus, מרדכי was ה׳תת אמה, 5800 amos tall. As mentioned in the pasuk, of course, this was only in the royal palace. The palace must have had very high ceilings, which is why the beginning of Ester, which describes the palace floors and other furnishings, doesn't describe the ceilings: anything more than twenty amos up is not noticed, as we see in hilchos Chanuka.
The Megillah says בַּיָּמִים הָהֵם, וּמָרְדֳּכַי יוֹשֵׁב בְּשַׁעַר-הַמֶּלֶךְ, "In those days, Mordechai would sit at the gate of the king" (Esther 2:21) Why was he sitting around doing nothing? He must have been the union contractor in charge of construction.
Now we know that א"ר חנינא חוזאה חזקה על חבר דאין מוציא מתחת ידו דבר שאינו מתוקן , a sage would never undertake a project that would violate building codes (Pesachim 9a). Now the palace in Shushan was modeled after the palace at Perseopolis, which was 19 meters in height. Using a conversion factor of .48 meters to an amah, the palace needed to be nearly 40 amos high.
In order to build such a palace in a safe and legal manner, Mordechai needed to grow accordingly, but only when he was on the premises. As pointed out in ba's answer, this explains why a 50 amah gallows was constructed for Mordechai.
It wasn't just Mordechai who was big. Ester 3:1 tells us the same thing about Haman. However, with Haman, it was not limited to when he was inside; it is unconditional, i.e. even outside. We find in Ester 7:9 that Haman was hanged on the gallows 50 amos high, so he couldn't have been taller than 50 amos. Just like the word "גדול" by Haman refers to no more than 50 amos, so too by Mordechai it is no more than 50 amos.
