What word sounds similar to "magnanimous" but means "impressively large"?

Solution 1:

Magnificent?

Impressively beautiful, elaborate, or extravagant; striking
_{^{Oxford Dictionary}}

It doesn't really imply anything about size, but it does derive from magnus.

There is (but it isn't in common use).

Magnitudinous: The adjectival form of magnitude.

Having the quality of greatness in size, amount, importance, etc.
_^Wordnik