Step 1: Comparing the three principal RNA types involved in protein synthesis by their typical length helps answer this question: mRNA length tracks the size of the protein-coding gene it is transcribed from, so it can run into thousands of nucleotides for large proteins.
Step 2: rRNA as a class includes very large molecules that form the core of ribosomal subunits, so even though a small rRNA species like 5S rRNA exists, the class overall skews large.
Step 3: tRNA, in contrast, is a uniformly compact adaptor molecule of only about 75 to 90 nucleotides across virtually all its varieties, folding into the characteristic cloverleaf structure. This consistent small size is why tRNA is generally considered the smallest of the major RNA classes.