The word "sizeableness" refers to the quality of being large or significant. There are several synonyms for this word that can be used interchangeably. The most common synonym is "magnitude," which refers to the great size or extent of something. Another synonym for "sizeableness" is "proportion," which refers to the relationship between different parts of something. "Amplitude" is also a synonym, which refers to the magnitude of something's motion or oscillation. "Extent" and "bulk" are often used as synonyms as well, both denoting the largeness or size of something. "Majesty" and "grandeur" could also be used in certain contexts to convey the sense of impressive size or impact.