diff --git a/lib/derivative_rodeo/generators/thumbnail_generator.rb b/lib/derivative_rodeo/generators/thumbnail_generator.rb
index 480dd16..70b3bd9 100644
--- a/lib/derivative_rodeo/generators/thumbnail_generator.rb
+++ b/lib/derivative_rodeo/generators/thumbnail_generator.rb
@@ -6,12 +6,33 @@ module Generators
     # This generator is responsible for converting a given binary into a thumbnail.  As of
     # <2023-05-22 Mon>, we're needing to generate thumbnails for PDFs and images.
     class ThumbnailGenerator < BaseGenerator
+      ##
+      # @!group Class Attributes
+
       ##
       # We want to mirror the same file "last" extension as described in Hyrax.
       #
       # @see https://github.com/samvera/hyrax/blob/426575a9065a5dd3b30f458f5589a0a705ad7be2/app/services/hyrax/file_set_derivatives_service.rb
       self.output_extension = 'thumbnail.jpeg'
 
+      ##
+      # @!attribute dimensions_by_type
+      #
+      #   @return [Hash<Symbol,String>] the "types" (as categorized by
+      #           Hyrax::FileSetDerivativeService).  These aren't mime-types per se but a conceptual
+      #           distillation of that.
+      #
+      #   @see https://github.com/samvera/hyrax/blob/815e0abaacf9f331a5640c5d6129661d01eadf75/app/services/hyrax/file_set_derivatives_service.rb
+      class_attribute :dimensions_by_type, default: { pdf: "338x493" }
+
+      ##
+      # @!attribute dimensions_fallback
+      #
+      #   @return [String] when there's no match for {.dimensions_by_type} use this value.
+      class_attribute :dimensions_fallback, default: "200x150>"
+      # @!endgroup Class Attributes
+      ##
+
       ##
       # @param output_location [StorageLocations::BaseLocation]
       # @param input_tmp_file_path [String] the location of the file that we can use for processing.
@@ -23,6 +44,25 @@ def build_step(output_location:, input_tmp_file_path:, **)
         end
       end
 
+      ##
+      # @param filename [String]
+      # @return [String]
+      #
+      # @see .dimensions_by_type
+      # @see .dimensions_fallback
+      #
+      # @note TODO: This is a very quick and dirty and assumptive type detector.  For the 2023-05-31
+      #       use case it is likely adequate (e.g. if it ends in .pdf we'll have a configured
+      #       match).  In other words, we'd love someone else to be sniffing out mime-types rather
+      #       than doing it here.
+      def self.dimensions_for(filename:)
+        type = filename.split(".")&.last&.to_sym
+        dimensions_by_type.fetch(type, dimensions_fallback)
+      end
+
+      # Want to expose the dimensions_for as an instance method
+      delegate :dimensions_for, to: :class
+
       ##
       # Convert the file found at :path_to_input into a thumbnail, writing it to the
       # :path_for_thumbnail_output
@@ -30,8 +70,8 @@ def build_step(output_location:, input_tmp_file_path:, **)
       # @param path_of_file_to_create_thumbnail_from [String]
       # @param path_for_thumbnail_output [String]
       def thumbnify(path_of_file_to_create_thumbnail_from:, path_for_thumbnail_output:)
-        # @todo the dimensions might not be always 200x150, figure out a way to make it dynamic
-        `convert #{path_of_file_to_create_thumbnail_from} -thumbnail '200x150>' -flatten #{path_for_thumbnail_output}`
+        dimensions = dimensions_for(filename: path_of_file_to_create_thumbnail_from)
+        `convert #{path_of_file_to_create_thumbnail_from} -thumbnail '#{dimensions}' -flatten #{path_for_thumbnail_output}`
       end
     end
   end
diff --git a/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb b/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb
index cb583e9..586b04e 100644
--- a/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb
+++ b/spec/derivative_rodeo/generators/thumbnail_generator_spec.rb
@@ -8,6 +8,36 @@
     it { is_expected.to eq("thumbnail.jpeg") }
   end
 
+  context '.dimensions_by_type' do
+    subject { described_class.dimensions_by_type }
+    it { is_expected.to be_a(Hash) }
+  end
+
+  context '.dimensions_fallback' do
+    subject { described_class.dimensions_fallback }
+    it { is_expected.to eq("200x150>") }
+    it { is_expected.to be_a String }
+  end
+
+  describe '.dimensions_for' do
+    subject { described_class.dimensions_for(filename: filename) }
+
+    context "given a file ending in '.pdf'" do
+      let(:filename) { "really-cool.pdf" }
+      it { is_expected.to eq described_class.dimensions_by_type.fetch(:pdf) }
+    end
+
+    context "given a file without an extension" do
+      let(:filename) { "aint-no-extension-here" }
+      it { is_expected.to eq described_class.dimensions_fallback }
+    end
+
+    context "given a file ending in '.tiff'" do
+      let(:filename) { "muppet-man.tiff" }
+      it { is_expected.to eq described_class.dimensions_fallback }
+    end
+  end
+
   describe "#generated_files" do
     context 'for a PDF' do
       it 'creates a thumbnail.jpeg file' do