The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-<em>a</em> concentration (pico- <2 <em>μ</em>m, nano- 2–20 <em>μ</em>m, and microphytoplankton >20 <em>μ</em>m) obtained using <em>in vitro</em> fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and <em>in situ</em> radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-<em>a</em>, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-<em>a</em> algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-<em>a</em>. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-<em>a</em> among the three size classes as a function of total chlorophyll-<em>a</em>. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-<em>a</em> during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.