Following is the code @Ray Carnes

account_id = fields.Many2one(
    'account.account',
    string='Fixed Asset Account',
    help="Select the fixed asset account."
)
capacity_form = fields.Selection(
    selection=[],
    string='Capacity',
    help="Select the capacity for the selected fixed asset account."
)

@api.onchange('account_id')
def _onchange_account_id(self):
    if self.account_id:
        # Fetch all capacities related to the selected account
        asset_records = self.env['account.asset'].search([
            ('account_asset_id', '=', self.account_id.id)
        ])
        # Prepare the selection list
        capacities = [(asset.capacity, asset.capacity) for asset in asset_records if asset.capacity]
        print('Capacities are', capacities)

        # Dynamically update the selection options for capacity_form
        self._fields['capacity_form'].selection = capacities
        self.capacity_form = capacities

        # Reset the value of capacity_form if it doesn't match the new options
        if self.capacity_form not in dict(capacities):
            self.capacity_form = False
    else:
        # Clear the selection if no account_id is selected
        self._fields['capacity_form'].selection = []
        self.capacity_form = False