"medieval" is quite a long time-span and of course sources are unreliable, but battles as large as in the movies certainly did happen. The battle of Agincourt is often estimated to 4 to 8 thousand english against 12 to over 36thousand frenchmen. The largest on european surface is often named as the battle of Grunwald/Tannenberg in which the estimations reach as high as 65 thousand. But these standoffs were rare.

To give you a perspective: The hundred year's war saw only 5 - 7 battles that exceeded 30000 soldiers. The "lesser" battles often ranged between 5 and 20 thousand soldiers.

Sources: Anne Curry, The Hundred Years War; stephen turnbull Tannenberg 1410 ( I listed english for Tannenberg although my own were german ones, but I don't know if those are available in english, the Curry book is really good though)

You might be interested to know that the scale of battles in the Ancient era was larger than anything that would be seen again until say, the late medieval era or beyond, even.

The battles of Alexander the Great, the Persian invasion of Greece, and the Punic Wars featured battles reaching into the hundreds of thousands of men. This is even accounting for the occasional propensity of ancient sources to embellish their claims.