diff --git a/src/fsfw/cfdp/VarLenFields.h b/src/fsfw/cfdp/VarLenFields.h
index 3143a857..69835c37 100644
--- a/src/fsfw/cfdp/VarLenFields.h
+++ b/src/fsfw/cfdp/VarLenFields.h
@@ -25,7 +25,7 @@ class VarLenField : public SerializeIF {
   template <typename T>
   explicit VarLenField(UnsignedByteField<T> byteField);
 
-  VarLenField(cfdp::WidthInBytes width, size_t value);
+  VarLenField(cfdp::WidthInBytes width, uint64_t value);
 
   bool operator==(const VarLenField &other) const;
   bool operator!=(const VarLenField &other) const;
@@ -44,7 +44,7 @@ class VarLenField : public SerializeIF {
                             Endianness streamEndianness);
 
   [[nodiscard]] cfdp::WidthInBytes getWidth() const;
-  [[nodiscard]] size_t getValue() const;
+  [[nodiscard]] uint64_t getValue() const;
 
 #if FSFW_CPP_OSTREAM_ENABLED == 1
   friend std::ostream &operator<<(std::ostream &os, const VarLenField &id) {
diff --git a/src/fsfw/filesystem/HasFileSystemIF.h b/src/fsfw/filesystem/HasFileSystemIF.h
index 3f7088d6..4f041135 100644
--- a/src/fsfw/filesystem/HasFileSystemIF.h
+++ b/src/fsfw/filesystem/HasFileSystemIF.h
@@ -80,7 +80,7 @@ class HasFileSystemIF {
 
   virtual bool isDirectory(const char* path) = 0;
 
-  virtual bool getFileSize(FilesystemParams params, size_t& fileSize) = 0;
+  virtual bool getFileSize(FilesystemParams params, uint64_t& fileSize) = 0;
 
   virtual bool fileExists(FilesystemParams params) = 0;
 
diff --git a/src/fsfw_hal/host/HostFilesystem.cpp b/src/fsfw_hal/host/HostFilesystem.cpp
index e2f8d808..b574c6c0 100644
--- a/src/fsfw_hal/host/HostFilesystem.cpp
+++ b/src/fsfw_hal/host/HostFilesystem.cpp
@@ -185,7 +185,7 @@ ReturnValue_t HostFilesystem::getBaseFilename(FilesystemParams params, char *nam
   return returnvalue::OK;
 }
 
-bool HostFilesystem::getFileSize(FilesystemParams params, size_t &fileSize) {
+bool HostFilesystem::getFileSize(FilesystemParams params, uint64_t &fileSize) {
   if (!fileExists(params)) {
     return false;
   }
diff --git a/src/fsfw_hal/host/HostFilesystem.h b/src/fsfw_hal/host/HostFilesystem.h
index 1946ab9a..fd745e09 100644
--- a/src/fsfw_hal/host/HostFilesystem.h
+++ b/src/fsfw_hal/host/HostFilesystem.h
@@ -11,7 +11,7 @@ class HostFilesystem : public HasFileSystemIF {
 
   ReturnValue_t getBaseFilename(FilesystemParams params, char *nameBuf, size_t maxLen,
                                 size_t &baseNameLen) override;
-  virtual bool getFileSize(FilesystemParams params, size_t &fileSize) override;
+  virtual bool getFileSize(FilesystemParams params, uint64_t &fileSize) override;
   bool isDirectory(const char *path) override;
   bool fileExists(FilesystemParams params) override;
   ReturnValue_t truncateFile(FilesystemParams params) override;