Is exact size important for barcode symbols?
For some types of barcodes it is important. For ITF-14 size is required to be 142.75 mm wide and 32 mm high. For EAN-13 it is required to be 37.29 mm wide and 27.85 mm high.
Can barcode be very small?
It can be reasonably small. For example 1x1 cm for 2D barcode. But in the smaller barcode you can encode less information. And you also should use a printer with higher resolution.
How much data can barcode contains?
It is usual to be about 25 characters for 1D barcode and about 2000 for 2D one. Of course the more characters you encode the bigger is barcode. 1D barcode could became impractically wide if it is more than 15 characters encoded.
What is a 1D or linear barcode? What is a 2D barcode?
A 1D or linear barcode is a combination of vertical bars. For some of them you can encode only numbers for other you can encode letters also. Most of barcode scanners can read them scanning just a line. 2D barcode encodes information in two dimensions and therefore can keep a way more information in it. And it is usually smaller than 1D one. More complex scanner is required to read this type of barcodes.
What is the best barcode to use?
Different industries have standards of barcode types to use. For labeling products in retail EAN and UPC are used. Use ISBN, ISSN & Bookland for books and periodicals. Use UCC-128, EAN-128 or SSCC-18 for labeling shipping boxes. Note that industry standard is not necessarily the same as the barcode symbology. ISBN, ISSN are undustry standarts that are based on EAN barcode symbology. UCC-128, EAN-128 and SSCC-18 use Code 128 barcode. In general if the items are to be scanned or sold via retail then the best barcode type to use would be the UPC or EAN barcodes. Consider also the amount of data that you require, if alphanumeric is required then Code 39 or code 128 is the best option. Also consider the size of the items you are barcoding and the system you will be using as 2D barcodes are much smaller but may not be supported by your system or that of your supply chain partners.