[pickers] Add inputSizeSmall to classes interface (#18242)

Co-authored-by: Lukas <llukas.tyla@gmail.com>

Author: sai6855

packages/x-date-pickers/src/PickersTextField/PickersInputBase/pickersInputBaseClasses.ts

@@ -12,6 +12,8 @@ export interface PickersInputBaseClasses {
   readOnly: string;
   /** State class applied to the root element if `error=true`. */
   error: string;
+  /** State class applied to the root element if `size=small`. */
+  inputSizeSmall: string;
   /** Styles applied to the NotchedOutline element. */
   notchedOutline: string;
   /** Styles applied to the real hidden input element. */
@@ -52,6 +54,7 @@ export const pickersInputBaseClasses = generateUtilityClasses<PickersInputBaseCl
     'adornedStart',
     'adornedEnd',
     'input',
+    'inputSizeSmall',
     'activeBar',
   ],
 );