diff --git a/README.md b/README.md
index ff4e8b9..a37befe 100644
--- a/README.md
+++ b/README.md
@@ -6,8 +6,8 @@
 To use this library you must to provide functions for both reading and writing the I2C bus. Function definitions must be the following.
 
 ```c
-int32_t i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size);
-int32_t i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, uint16_t size);
+int i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size);
+int i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, size_t size);
 ```
 
 Where `address` is the I2C address, `reg` is the register to read or write, `buffer` holds the data to write or read into and `size` is the amount of data to read or write. The `handle` parameter is an optional customizable argument. You can use it if your I2C implementation requires any additional information such has number of the hardware I2C driver. For example function implementations see [ESP I2C helper](https://github.com/tuupola/esp_i2c_helper). For working example see [M5StickC kitchen sink](https://github.com/tuupola/esp_m5stick).
diff --git a/bm8563.h b/bm8563.h
index f2827e0..0a43be1 100644
--- a/bm8563.h
+++ b/bm8563.h
@@ -97,8 +97,8 @@ extern "C" {
 
 /* These should be provided by the HAL. */
 typedef struct {
-    int32_t (* read)(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size);
-    int32_t (* write)(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, uint16_t size);
+    int (* read)(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size);
+    int (* write)(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, size_t size);
     void *handle;
 } bm8563_t;
 
diff --git a/tests/mock_i2c.c b/tests/mock_i2c.c
index b0336b0..14208d0 100644
--- a/tests/mock_i2c.c
+++ b/tests/mock_i2c.c
@@ -39,8 +39,8 @@ SPDX-License-Identifier: MIT
 
 uint8_t memory[255] = {0};
 
-int32_t
-mock_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size)
+int
+mock_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size)
 {
     (void) handle;
     (void) address;
@@ -48,8 +48,8 @@ mock_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint1
     return BM8563_OK;
 }
 
-int32_t
-mock_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, uint16_t size)
+int
+mock_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, size_t size)
 {
     (void) handle;
     (void) address;
@@ -57,8 +57,8 @@ mock_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer
     return BM8563_OK;
 }
 
-int32_t
-mock_i2c_low_voltage_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size)
+int
+mock_i2c_low_voltage_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size)
 {
     (void) handle;
     (void) address;
@@ -67,8 +67,8 @@ mock_i2c_low_voltage_read(void *handle, uint8_t address, uint8_t reg, uint8_t *b
     return BM8563_OK;
 }
 
-int32_t
-mock_failing_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size)
+int
+mock_failing_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size)
 {
     (void) handle;
     (void) address;
@@ -78,8 +78,8 @@ mock_failing_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffe
     return MOCK_I2C_ERROR;
 }
 
-int32_t
-mock_failing_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, uint16_t size)
+int
+mock_failing_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, size_t size)
 {
     (void) handle;
     (void) address;
diff --git a/tests/mock_i2c.h b/tests/mock_i2c.h
index 6888478..ecded2d 100644
--- a/tests/mock_i2c.h
+++ b/tests/mock_i2c.h
@@ -35,10 +35,10 @@ SPDX-License-Identifier: MIT
 
 #define MOCK_I2C_ERROR  (3)
 
-int32_t mock_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size);
-int32_t mock_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, uint16_t size);
+int mock_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size);
+int mock_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, size_t size);
 
-int32_t mock_i2c_low_voltage_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size);
+int mock_i2c_low_voltage_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size);
 
-int32_t mock_failing_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, uint16_t size);
-int32_t mock_failing_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, uint16_t size);
+int mock_failing_i2c_read(void *handle, uint8_t address, uint8_t reg, uint8_t *buffer, size_t size);
+int mock_failing_i2c_write(void *handle, uint8_t address, uint8_t reg, const uint8_t *buffer, size_t size);