For example, there is Product model below:
class Product(models.Model):
    name = models.CharField(max_length=50)
    description = models.TextField() 
    price = models.DecimalField(decimal_places=2, max_digits=5)
    quantity = models.PositiveSmallIntegerField()
    
    def __str__(self):
        return self.name
Then, using formfield_overrides or form below.
formfield_overrides:
# "admin.py"
from django.contrib import admin
from .models import Product
from django.db import models
from django import forms
@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):
    formfield_overrides = { # Here
        models.CharField: {
            'widget': forms.TextInput(attrs={'size':'50'})
        },
        models.TextField: {
            'widget': forms.Textarea(attrs={'rows':'1', 'cols':'50'})
        },
        models.DecimalField: {
            'widget': forms.NumberInput(attrs={'style': 'width:50ch'})
        },
        models.PositiveSmallIntegerField: {
            'widget': forms.NumberInput(attrs={'style': 'width:50ch'})
        },
    }
form:
# "admin.py"
from django.contrib import admin
from .models import Product
from django.db import models
from django import forms
class ProductForm(forms.ModelForm):
    class Meta:
        widgets = {
            'name': forms.TextInput(attrs={'size':'50'}),
            'description': forms.Textarea(attrs={'rows':'1', 'cols':'50'}),
            'price' : forms.NumberInput(attrs={
                'style': 'width:50ch'
            }),
            'quantity' : forms.NumberInput(attrs={
                'style': 'width:50ch'
            })
        }
@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):
    form = ProductForm # Here
form:
# "admin.py"
from django import forms
from django.contrib import admin
from .models import Product
class ProductForm(forms.ModelForm):
    name = forms.CharField(
        widget=forms.TextInput(attrs={'size':'50'})
    )
    description = forms.CharField(
        widget=forms.Textarea(attrs={'rows':'1', 'cols':'50'})
    )
    price = forms.DecimalField(
        widget=forms.NumberInput(attrs={'style':'width:50ch'})
    )
    quantity = forms.IntegerField(
        widget=forms.NumberInput(attrs={'style':'width:50ch'})
    )
@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):
    form = ProductForm # Here
You can change the size of the fields in "Add" and "Change" pages as shown below:


In addition, in this case of only formfield_overrides, you can change the size of the fields in "Change List" page as shown below:
# "admin.py"
from django.contrib import admin
from .models import Product
from django.db import models
from django import forms
@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):
    list_display = ('name', 'description', 'price', 'quantity')
    list_editable = ('name',) # Here
    list_display_links = ('description',)
    formfield_overrides = { # Here
        models.CharField: {
            'widget': forms.TextInput(attrs={'size':'50'})
        },
        models.TextField: {
            'widget': forms.Textarea(attrs={'rows':'1', 'cols':'50'})
        },
        models.DecimalField: {
            'widget': forms.NumberInput(attrs={'style': 'width:50ch'})
        },
        models.PositiveSmallIntegerField: {
            'widget': forms.NumberInput(attrs={'style': 'width:50ch'})
        },
    }
