Hello @albriro,
For PayPal button shape in checkout page navigate to: (line 46)
wp-content/plugins/woocommerce-gateway-paypal-powered-by-braintree/includes/payment-forms/class-wc-braintree-paypal-payment-form.php
For PayPal button shape in cart page navigate to: (line 141)
wp-content/plugins/woocommerce-gateway-paypal-powered-by-braintree/includes/class-wc-braintree-paypal-cart.php
$default_button_styles = array(
label: 'checkout', // pay | paypal | buynow | checkout | credit
size: 'responsive', // small | medium | large | responsive
shape: 'rect', // pill | rect
color: 'gold', // gold | blue | silver | black
tagline: false, // true | false
fundingicons: true, // true | false
);