Use Get Product endpoint? Look for base_increment and base_min_size.
{"product_id":"SOL-GBP","price":"52.79","price_percentage_change_24h":"-7.45091164095372","volume_24h":"34107.541","volume_percentage_change_24h":"-13.01018448475413","base_increment":"0.001","quote_increment":"0.01","quote_min_size":"1","quote_max_size":"10000000","base_min_size":"0.08","base_max_size":"60000","base_name":"Solana","quote_name":"British Pound","watched":false,"is_disabled":false,"new":false,"status":"online","cancel_only":false,"limit_only":false,"post_only":false,"trading_disabled":false,"auction_mode":false,"product_type":"SPOT","quote_currency_id":"GBP","base_currency_id":"SOL","fcm_trading_session_details":null,"mid_market_price":"","alias":"","alias_to":[],"base_display_symbol":"SOL","quote_display_symbol":"GBP","view_only":false,"price_increment":"0.01"}
Looks like minimum size is 0.08 and max precision you can use is up to 3 decimal places.
In Pro minimum size limit was removed, you can check this.