Fucoidan is a complex-sulfated polysaccharide distributed in various marine organisms, and the brown algae are reported as the major producer. The fucoidan is important for their high bioactive properties, like antibacterial, anticoagulant, antiviral, anti-tumor, etc., and many more to be explored. There is a strong archival support for the bioactivity and promising properties of this molecule, which creates a hope for this molecule as future drug against thrombosis and some kind of cancers. Reports other than the above bioactive properties have also been a matter of interest for the design of signal or enzyme-arrested new class of drugs. In the past three decades, the research on isolation, molecular characterization, and screening of biological applications has significantly increased. One major issue associated with this molecule is the higher size and seasonal variation in their chemical composition; to resolve the issue and maintain its bioactivity, a prioritized and literal hydrolysis process is required to be developed. Here, in this mini-review, we have tried to summarize the algal fucoidan research and the bioactivities influenced by their molecular size.