Hi FLecointe,
Thanks for contacting Syncfusion Support.
We can customize the SfPicker dimensions by using PickerWidth,PickerHeight properties and we can also customize the picker font size by using SelectedItemFontSize ,UnselectedItemFontSize, HeaderFontSize properties. We have prepared sample using this properties. Please find the sample from the below link.
If the above sample does not meet your requirement can you please share more details like Screenshot, platform in which you are facing the issue and if possible modify the attached sample so that we can help you achieving the requirement
Please let us know if you have any other query.
Regards,
Rathanakumar S.