[docs] Add table for size of builtin types

Author: tobiashienzsch

docs/design.md

@@ -1,5 +1,27 @@
 # Design
 
+## Builtin Types
+
+|       **Type**       | AVR | MSP430 | ARM32 Thumb | Unix ARM64 | Win ARM64 | Unix X64 | Win X64 | WebAssembly |
+| :------------------: | :-: | :----: | :---------: | :--------: | :-------: | :------: | :-----: | :---------: |
+|   `unsigned char`    |  1  |   1    |      1      |     1      |     1     |    1     |    1    |      1      |
+|   `unsigned short`   |  2  |   2    |      2      |     2      |     2     |    2     |    2    |      2      |
+|    `unsigned int`    |  2  |   2    |      4      |     4      |     4     |    4     |    4    |      4      |
+|   `unsigned long`    |  4  |   4    |      4      |     8      |     4     |    8     |    4    |      4      |
+| `unsigned long long` |  8  |   8    |      8      |     8      |     8     |    8     |    8    |      8      |
+|       `float`        |  4  |   4    |      4      |     4      |     4     |    4     |    4    |      4      |
+|       `double`       |  4  |   8    |      8      |     8      |     8     |    8     |    8    |      8      |
+|    `long double`     |  8  |   8    |      8      |     16     |     8     |    16    |    8    |      8      |
+|        `char`        |  1  |   1    |      1      |     1      |     1     |    1     |    1    |      1      |
+|      `wchar_t`       |  2  |   4    |      4      |     4      |     2     |    4     |    2    |      4      |
+|      `char8_t`       |  1  |   1    |      1      |     1      |     1     |    1     |    1    |      1      |
+|      `char16_t`      |  2  |   2    |      2      |     2      |     2     |    2     |    2    |      2      |
+|      `char32_t`      |  4  |   4    |      4      |     4      |     4     |    4     |    4    |      4      |
+
+### AVR
+
+Size of `double` and `long double` can be changed with `-mdouble=bits` and `-mlong-double=bits`. Possible values are `32` and `64`.
+
 ## Issues
 
 ### Forced to use namesapce std